Ok, here's my take on it. Start back at the beginning, and small, and short. Is there another facility that you can board him at? Maybe one with a proper roundpen? You need to get his respect. Being sweet doesn't mean they are respectful. ;) Don't pay attention to his size, YOU be "bigger than him". If you start him in a small circle, watch his body language, if it looks like he's going to "explode" get after him before he does it. Or change direction. Make him do something YOU want him to do. Get him where he is paying attention to what you are going to tell him to do next, not "I'm bored with this let's do something stupid". LOL Now, I'm NOT into all the "gimmicks" from all of the different trainers. BUT, my neighbors got out of horses, gave me their left over tack. In it was a Monty Roberts training halter. Here's the link. Dually Halter Medium | Monty Roberts
OMG I have used this on all of my foals, (once they are yearlings). The results are amazing. They STOP any yanking, etc. They learn to lunge beautifully. Do lots of little circles, don't expand your circle until he masters the little one several different sessions. Always end on a good note. Even if you work him for 5 minutes.
I have an appendix QH (1/2 TB/ 1/2 QTR) mare that I show HUS. She very much takes after the TB side, in every way. I've found that "hot" feed, such as Alfalfa really affects her temperament. Maybe look into what he's being fed?
Good luck with your boy. You can do it! (just be careful) :)