I agree that the way to a horse's heart is through their stomach. When I used to ride PLP (Perfect Lesson Pony) a lot, I gave her cookies every time I rode her, before and after, and I tried to scrounge some alfalfa hay for her to eat while I groomed and tacked her. Aside from riding her, the only thing I ever did with her was sometimes give her a cookie if I was out in the pasture, and greet her when I was out in the pasture. And within a couple of months, she would come when I called, follow me around the pasture, and actually try to chase off my horses from being close to me (we had to put a stop to that obviously). Just from some cookies and a little hay. Well, and maybe because I treated her with the utmost kindness just in general also. But despite me being an absolutely TERRIBLE rider.
My own guys are generally easy keepers who only get feed (as opposed to hay) when I give it to them, and I don't give them a lot. But I do buy bales of alfalfa hay (maybe once or twice a month) from the barn owner and I give them a flake or half a flake when I bring them in. Moonshine in particular learned the routine very quickly so I don't even halter her any more, just call her in from the pasture and let her go to "her" stall where she paces around waiting for me to bring her the food that is her due. A little alfalfa hay every other day or a couple of times a week is a nice treat for them and isn't going to hurt them.
I like the idea of doing a little trick training before or after your ride. Most horses love it, mostly I suspect because it means they get treats. Some horses (cough cough my Pony) need to get treats only when they are doing a specific good thing, not just as a general treat, or they will get pushy. Which is why trick training is great. Because of my experience with Pony, I no longer give treats "just because" unless I know the horse won't get pushy. Even PLP got backed off cookies for a week when she got pushy.
One other thing if possible would be to go out there sometimes and just hang out with him. Bring him in and just groom him, give him nice hay, hang out, then turn him out again.
I totally feel you about having a horse in training and being a worse rider (in my case much worse) than the trainer. I thought that was going to happen with Pony, and maybe it did at first (for instance he used to pull all sort of pony tricks because he could get away with them with me on his back) but either because I've become a better rider or because he's gotten to know me better in the year that I've had him, he now actually tries harder for me than for others, even the trainer. So, based on my experience, I would say that is just a phase in your relationship. You guys already have a level of bonding. Things will get better as long as you keep working on them.