Yep! And teach energy appropriately. Don’t expect that he can maintain an energetic trot on his own for even 1 lap of the ring for now. He’s lived a “western pleasure” life for a while now. I don’t now if you’ve just taken it for granted, or half-heartedly tried to get more but give up, but either way, it’s his normal right now.
You don’t want to do “damage” to his mental outlook of work by suddenly whipping him into shape (metaphorically).
Teach him to get HOT off your leg first. Grab mane because if he decides to “I’ll show you!!” and bolts off, the last thing you want is to grab his mouth. If he does that, quietly bring him back and praise the crap out of him. Modify your next ask 
Once he’s HOT off your leg - as in, immediately and energetically but also respectfully - THEN start asking him to maintain that for a bit longer, and longer, and longer.
And keep these rides short. This work is going to be physically harder for him, and for sure mentally harder. 15 minutes is totally appropriate at the beginning. This sort of thing should be all you work on for a few weeks if necessary. GO. Go NOW.
Then you can start in with maintaining that increased pace - not leg speed, that’s rushing and not what you want.
And I tell you, this means you need to be working on this “Go, Go NOW” thing with every interaction. You put the halter on to lead him somewhere? He needs to get right there with you. Use a dressae whip behind you, on his side, as your “leg”, do not pull on the lead rope. All this movement has to originate from his hind end, always. You can spend 1-2 days a week even doing ground sessions teaching him that an ask for energy means he needs to go YES M’AM, but with thought and not just blindly reacting - does that make sense?
You get on and you need to make sure that you are ready to ask him to MOVE off the moutning block. Not bolting (but again, grab mane), but he needs to learn that those first steps are not front feet first pulling him into movement at his leisure 
So very time, make sure you are ready to ask him properly, be ready for an exaggerated reaction, but also be ready to escalate your asking until you get a reaction and then QUIT asking.
These sluggy guys may need to be ridden with your leg totally off them for now, if part of the issue is they have become dead to the leg. Teach their sides how to be left alone when doing as askedm which makes leg on a more loud communication. Then you work your way back to normal leg resting on his side.
The better your consistency, the faster this will all go. And it’s worth it 