He could not like the bit
I may work on lunging and noticing if he has self carriage or not, if not I would do plenty of lunging and working on keeping him relaxed and even so he can stretch his top line and work on that self carriage! Good luck:)
Make sure that you keep your leg on when asking your horse to be on the bit. When he puts his head down even a little bit, give with your elbows so he relaxes and understands that what he was doing was right. Also pats and "good boy!"s always are a good idea. Starting with doing this at the walk and then practicing the same idea over poles at the trot once he accepts contact at the walk would be a good idea. I don't ride western but I believe the concept is the same. Good luck, and hope I could help!
Be light handed and does he get nervous around the arena? If so warm him up far from the arena and stay back from the arena until it's your time to run. If your and home go on trail rides stay away from the arena
Make sure it's the right size for him
Perhaps he has an injury or something is causing pain when he collects which you might not notice when he is moving normally or sometimes it can be due to lack of balance