I’d agree with a lot of what other posters have said: the Appy has no (or not enough) respect for you and that needs to change.
Personally, I don’t think that sending the horse to a trainer will do much for this situation unless you’re willing to go to the trainer often enough and get trained yourself on how to deal with the horse to make his newly learned respect truly stick.
The horse, and every other horse in your pasture too, has to learn that when you are around, the horse has to be aware of where you are at all times and has to do his utmost to avoid harming you, even accidentally or incidentally.
The horse’s attitude towards you might improve with some significant and focused groundwork with him. Simple things, really, like leading properly, matching your steps, stopping when you stop, learning to stand on command while you walk away. But all of these simple things should be done with purpose and with him 100% focused on you. He doesn’t get to barge into your space (even just the slightest bit, even just with a shoulder); you back him off and when he does back up, let up the pressure and praise him. He has to learn to read you, whenever you’re around and take care of you. (I don’t mean this in a sappy way, just that he has to recognize that you are not another horse and he needs to be mindful).
If he does come at you with intent, or if he does just keep barging into you, make yourself big and, as others have said, give him the worst 15 seconds of his life. Be careful with this, though: if he is truly an aggressive horse (as opposed to an opportunistic one), escalating the situation by going after him could provoke him to do worse. You’ve got to be able to read the horse.
If he is aggressive then he should go to the trainer and not come back.
Good luck.