Maybe you just need some direction. It sounds like he might be bored because you may just be at a loss of what to do when riding him, you know?
I'm not sure of your riding experience or how long you've had him or anything.. but I think if you had a goal in mind.. and thought of creative ways to reach those goals.. you'd probably do much better.
He sounds kind of like he's comfortable plodding along and when he gets excited it's because it's new and interesting. When you ask for more, he probably does the horsey equivalent of shrugging and you take that as he's tired or low key?
It may be a good idea to find a trainer that teaches something you've always wanted to learn.. like jumping for example, or hunters, or whatever.. and work with them. Not so much to learn, but you know something new.
I don't think changing his feed to get him hot is a good idea.. but maybe his current feed isn't filling all his needs. Maybe he's deficient in something or working hard but not getting enough calories, etc.
Have you talked with your vet about this? Since he knows the horse better than me :P
"Strength is the ability to use a muscle without tension"