Really looks like you're using his mouth to catch yourself (even when not jumping), big nono. I would recommend you do more flat work, learn to ride your seat better before jumping, neither you or the horse are ready for it.
Jumping on the "STOP JUMPING A 3 YEAR OLD" wagon, much too young imo to have him going like that, you risk ruining him physically and mentally.
Not having a trainer can be fine, but you need to go slower and realize that you might not be able to ever jump properly without one. Get back to the flat work and slow down, this colt is not ready for what you are asking him, and neither are you.