To start off, go back to just all-the-time lessons on "space". You should have a bubble of personal space around you, maybe a couple of feet, that he is NOT allowed to enter, ever. You are allowed to enter HIS space, but he should not poke his nose or his shoulder or his hoof into yours. Horses play "made-you-move", where every subtle chance they get, they get just a little into your space and make you move over. This is a small victory for them and each time it makes them respect you less. So, if your boy ever tries to get into your space (especially at feeding time), IMMEDIATELY move him over, with your hand or a lead rope or even a whip. Move him back, around, just make him MOVE. This will establish you as leader. Also, a really great ground training method that will get you as leader in just about a half-hour is Monty Roberts' method. He wrote the book The Man Who Listens To Horses
which explains it all, and also has a website, www.montyroberts.com
, I believe. I've tried the method with all of my horses, including minis, and it really makes them respect and trust you in a very short period of time (can be done in one training session if you do it right).
For the bit...I totally agree about the tom thumb, they should never be used on young horses. Our little trail mustang had his mouth permenantly hardened by one, although he wore it for much longer then your boy. They are better for more advanced horses that know how to neckrein already and whose riders want their mouths to be super-sensitive to feel every hand movement. You might want to try a slow-twist eggbutt snaffle; it has a bit of a twist to it that just gives a bit of "grab" to get his attention, without any extra leverage. You can get a full-cheek in slow-twist too, which, as Draftrider said, will aid with turning. Full cheeks sometimes tend to make horses stiff in their necks, though, so don't use it for too long. You can get half-cheeks, too, which will have the same turning affect, just more mildly. After he is listening, you can put him in a loose-ring snaffle, which will encourage him to relax and salivate. Certain mouthpieces (copper, "sweet metal") claim to encourage salivation, again which will get him to relax and listen.
And the feed...put him on grass hay first, a lot if necessary, and if he is still looking skinny add some alfalfa and maybe some grain, but not too much, as it sounds like he is enough horse already without being hyper for grain. Plus, hay is really good for their digestion in a lot of ways. So I would keep him on hay until he gains his weight back, then add some grain after you get him into full work (we feed just pure oats) and you can put some corn, vegetable or special cocasoya oil in it to make his coat healthy and shiny and to help him keep his weight. Soybean meal can aid with weight gain, too, although when we tried it on our TB it gave him really soft stools and made him lose
weight instead of gain.
So hope this helps...I'm sorry as it turned out every bit as long as yours. Good luck with re-training! :)