this horse isn't ready for all that you're doing. you're rushing him through things and not letting him think his way through. So, just take a step back, and do one thing at a time, until he's accepted that one thing, then add something else.
1) desensitizing: with approach and retreat, be able to touch and rub his face with the fly mask until he's just fine with it, before you actually put it on. if you can't even do that, then he shouldn't be wearing it til he's fine.
2) clearly he has no idea what pulling back on the reins means (to stop a horse you should NEVER pull on the reins anyway. you should ONLY take out the slack and wait for the horse to yield). you're trying to force this on him and he is either scared or frustrated and you hurt him with the excess pressure in his sensitive mouth.
**get a trainer to help you start him correctly** there's a whole process of first using one rein at a time to stop the horse (via a one rein stop) and your SEAT and legs and lightly your fingers.....it's not about ever pulling back.
If you can't ride him in a plain smooth bar snaffle, you've no business riding him in anything else. So, don't switch to any other bit before you have mastered the snaffle.
Also....the bit sounds too big for his mouth. Find a pony size or other size bit. If the bit can go under his tongue, it doesn't fit. OR you have it too loose (*all points to....you need a trainer)
3) don't jump a 4 year old horse that is too young and not conditioned enough for jumping. Jumping means a horse will land all of his weight on those 2 front legs = problems later on. Unless you want early on tendon, ligament and other medical problems...be considerate of his physical needs. he needs to grow up and needs conditioning and be sure to always use protection for his legs (sport medicine boots)
*I'd strongly suggest getting a trainer involved. You'll be better off with a professional to get your horse started right, otherwise, you'll find that your horse will develop really bad habits that you train him to get...that you don't want.