What kind of bit are you using on him? He may just not like it
I have a Nurtural No-Bit bitless
that I use for trail riding. I LOVE it! I also have a treeless saddle (a bob marshall). I still use my bits for training, but I like using the bitless for trails and "fun" rides. Why use a bit if you don't have to?
When you're working, try lifting the reins straight up when you cue with your legs. Keep lifting up until he brings his head up, then release your cues and praise him.