Aggression is a dangerous vice. No way to sugarcoat that. If you are not safe with him, there is a serious problem. Even if he is not aggressive when food is around, if he thinks of you as the food source, he should not be aggressive if you don’t have any. Perhaps your trainer (I misread your pronouns initially, which confused me about which he you meant – we need more pronouns in this language) could help you with this issue, as he is probably larger and stronger than you. Good ground manners are essential – teach this horse that he does not run the planet by making him back up when you enter his stall and stand. He should know those words. Also, stop him from entering or leaving until you say to do so (literally stop him outside the stall and don’t let him in until you release him from the hold back). Require him to turn his hindquarters away from you so you can remove his halter if you are just leaving him in there, and make him stand quietly for you to groom him, if that is what you choose to do. As for treats, there are two schools of thought about them. Some people don’t like giving them at all, some give them as a reward for good behavior (that is what I do). I spent months teaching my horse the concept of “put your head down”, because when I got him it was like trying to groom a giraffe. He only gets a treat after I have completely finished the grooming ritual or if he has to stretch for one (he wasn’t flexible at all initially). They outweigh us generally by a factor of 10, so will always win a physical fight – never let it get to that. I’d prefer to say teach him good manners, but for your safety while you are working on that, can you put him in crossties for grooming? What I did was to put Joe Joe in crossties with a lead attached to his halter (we were working on accepting being in the wash stall, which he thinks is a gas chamber) so that if he got jumpy I could take the lead rather than having to leap for his halter to calm him down. It is less startling to the horse that way. All I can do is offer some ideas that have worked for me, and hope they may help you. The stall is his space, but he must allow you to enter it and do whatever it is you go in there for without any fussing at all. I have read of using a boiled potato (still hot) to combat biting, but not sure it is practical. Put it under your sleeve (or wherever he mostly bites) and let him bite into it. He would burn his mouth, and decide that the consequences are not nice, thus he would stop that behavior. I don’t know if this works, it is only something I read eons ago. You, if you were to do this, would just ignore all that and continue with whatever you were doing.
It is never the horse's fault