How to get a lazy horse moving?

Why don't you want to use spurs??

Well, he's not my horse, I only ride him when I have the opportunity and his owner never rides him with spurs so he isn't used to it and when he sees something flashy he ticks out...
Plus, I know that the problem is only his stubborness and that wouldn't change with spurs. I was more wondering if there was a way to show him that it's more rewarding if he doesn't act lazy....

I've only ridden with spurs once and that was in a lesson on a horse whose owner INSISTED I ride in spurs. 
There are so many fundamental differences in English and Western.  Heels in the side to move a horse is more of a last resort, they should listen to leg pressure, imo.
Are you riding in a safe environment?  Are you used to some crow-hopping?  Do you know how to correctly use a crop? I personally only have so much leg strength (and patience) before I'll be using my crop.  Usually one time lets them know you mean business and gets them moving.  Might get a crow-hop out of it but that's no biggee.  At least there's energy in that!  I think he's taking advantage of you which makes me wonder about your experience.  Are you taking lessons to better your skills?  (I forgot)
He's an older horse and probably could teach you a lot.  Hope you come to terms with him!

About my experience: I rode for like a year when I was a kid, then I had a break and now ride again since over 6 years... And usually I don't have problems with my aids and I got a lot of slow horses to go forward, so that isn't the problem. I sometimes take lessons for tests and certificates but also ride horses of my friends or my lovely Grand Slam on my own. And I sometimes ride a horse who usually doesn't go forward at all but after 20 minutes of work with him I can get a nice working speed...
 
Usually I start with only squeezing my legs slightly, if the horse doesn't respond I tap him with my lower leg, if even then there's no response, I give him a light "kick" with my heel and if that doesn't work I pat him with my whip. I try to apply lightest aids possible and when the horse finally responds I repeat the same thing with lighter aids...
 
The problem with this horse is: we already are at the "whip-tap-stage" and he still doesn't respond immediately. I know and I don't ask him to walk like a TB since he's old but mostly the upward-transition to trot and keeping the trot is the problem... The transition to the canter is totally easy and his canter is a very good speed....
 
Once he's warmed up he walks ok but mostly in the first 15 minutes he's very lazy. But that problem disappears when we go on trails. So I think it is because he's bored of riding in the arena.
 
And I am also not a huge fan of spurs, never used them on Grand Slam and only using them if an instructor or a friend ask me to.

Gotcha. 
If he's moving okay on the trail and you're sure that the trot isn't causing him discomfort (like arthritis) I'd give him a good swat!  He's past the "light" stage.  He's  probably laughing inside at that... or saying "a little to the left.....ahhhh...."  []
And ALWAYS behind the saddle, right? Right there on his butt!
But then praise him when he gives you some good trotting.  Just suggestions/what I'd be trying.

There are so many fundamental differences in English and Western. Heels in the side to move a horse is more of a last resort, they should listen to leg pressure, imo.

[quote=hunterseat]

There are not as many differences as you think.  I ride with spurs off and on.  Ask with pressure first, slightly roll leg to get more heel (not spur) second and the third time they get bumped with the spurs.  Guess who watched my use spurs on a horse and help with my transitions with a barrel pattern------ a dressage instructor.
 
If you are not wanting to use spurs then I would get a bit more agressive with the crop, not abusive, agressive.  He if is laggy back then push him forward more into circles, flexing.  KEEP HIS FEET moving and his mind thinking.  That way he will think, ok I can move how she wants or REALLY have to move if I goof off.  Even without spurs a good boot from a heal can motive them pretty good.

I've written an article on my new website, *link removed* might help you.  You can find it by clicking Article on How to Motivate a Lazy Horse.  All of the information at my website is free, so don't hesitate to look at my Blog, Horse Articles, or Horse Videos when you're searching for information on training, showing, or judging hunters or hunter seat equitation--Best wishes, Anna Jane White-Mullin

Okay, so here's a tiny addition:
 
since it snowed here lately, I was forced to ride that horse in a closed arena (which I never did before, we were always working in the open arena). Ans I noticed that it really worked out well. I got him moving, he even took a BIG jump over a 60cm fence (I thought maybe that was a way to motivate him and believe it or not, it did...) So I had no problem on forward moving or anything.
What I also noticed is that he seemed to be lazy on the way to the closed arena (10 minutes on horse) but on the way back, after we did some canter-work and that jump, he was much more forward thinking and only occasionally fell back to his slow pace...
 
So what should I change in my way of working him in the open arena? And why do you think he only walks well in the closed one?
 
*love*

Simply watching a rider struggle to make a lazy horse work can be exhausting, never mind dealing with one yourself. Nagging horses into reluctant submission is commonplace, but what is the cause of this unwillingness and can it be improved? "The biggest thing causing a non-responsive horse, is the rider giving too many aids," states dressage rider and coach Gary Vander&nbsploeg emphatically. "We are taught as green riders to give lots of aids, that is, look busy on the horse, and we are not taught how to be passive. The worst thing we can do is to 'drive stronger' and 'grip with your legs' with these horses, because we are actually training them to be dull."
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Earl Nunes

One of my daughters ponies dennis who is pictured in my avatar is very lazy! But now he is a complete star. For a horse to remeber a faltwork sort of thing you will have to do it 7 times over different days in close time to each other. What you must do is kick kick kick. Dont stop kicking until you get what you want but kick with one leg at a time and keep swappign which one goes first. By the end of the lesson you should feel a vast improvement and the horse will need much less kicking than before. But by the sgtart of the next ride he will have forgotten so repeat and as i said above after seven times that horse will see you as the person who will make him go not let him get away with it. Hope that helped!

Move the horse's shoulders and hips, ask him for two-tracks (leg yield) and half-passes at the trot, back up, do turns on the forehand and on the haunches and so on.