Also,if you see that doens't evolve, I recommend you to ask for a vet check because maybe your horse is getting agressive because of illnes.
In terms of getting after him in the moment, sharp pulls on a lead line or reins are good because they are a vety clear way of telling them that what they did was bad, but only ever do this if you really have to as it can sometimes be too extreme.A less extreme thing that you could also try using your body language and voice more. A sharp hey and stomp towards that is usually enough to make a horse step back.
The main thing you need to do is regain respect. If you are ever going into his stall, be assertive and don't hesitate, walking straight up to him and putting a halter on. Then focus a lot on ground work. Try leading him around the arena. And ask him to halt, back up and walk on. You could also ask for turns on the forehand. When taking her halter off it might also help if you give her no option but to stand still, and don't finish taking the halter off until he stands.