Long post: please bear with me!

I recently was hired as a groom at a local boarding/training facility. One of my favorite horses there is an 8 year old Arabian stallion. Unfortunately, he's quite a monster. He's attacked other horses and is aggressive with humans (mostly biting, also kicking), which has created a general animosity toward him and the frequent use of stud chains and shouting to "show him who's boss." This is my first job around horses though I have extensive volunteer experience with abused/neglected horses at a rescue. As a short woman, I know that if it comes down to it, I am not gonna out-physical a horse.

Important info to consider is that he was placed at this facility in 2021 after a fire at the farm he was born at. He was supposed to go back to his home after they rebuilt, but his owner visited one time in 2022 and now is not paying her bills, so this stallion has been receiving only basic care (food, water, time on the walker).

My first encounter with him, he pulled back on the lead, reared up, tossed his head at me, etc, then let me walk him out--until we go to the stall door when he swiftly slammed his body into mine and shoved me against the door frame. Luckily someone else was close by and took over. The next time I was alone and decided to take a different approach when I noticed he was trembling and showing other signs of fear. Gentling him was quick and effective though he tried to halfheartedly bite me afterwards. Since then, almost no problems (outside of stubbornness and attention seeking). He's even started accepting treats from me and seeks affection when I walk by his stall and has made playful gestures. Till yesterday.

I came back from being out of town, and the rest of the stable was preparing for a huge show, which always makes the horses antsy. This stallion was pulling back on the lead again and trying to control the direction, but was starting to chill out and seemed fine until he whipped his neck and slammed the side of his face directly onto mine with enough force to twist the arm of my glasses and knock me clean off my feet onto my back. He didn't attack me at all once I was on the ground or run off, instead just stood there looking at me. After getting up and taking the lead rope again, I didn't react at all and just kept taking him to the walker with no further issues. He walked for an hour but kept stopping it at first as well until I'd come over and tell him to knock it off. When I started taking him off the walker, he was nipping at my hand everytime I'd reach for the clip, almost like a game, almost like a little kid. Once I put him back into his stall, he ran to the window, shoved his face through, and tried to nuzzle me.

He is obviously intelligent and understimulated. I adore him. The owner of the facility and I have spoken, and he's given me the go ahead to work with this stallion more 1:1 each week, not for show training or anything, but for building trust and mental stimulation. While I don't subscribe to the "show him who's boss" mentality, I'm also not an idiot, and I am firm with my boundaries and not going to put myself into a high-risk situation that I can't handle.

After thinking about yesterday's behavior, I'm wondering if it might have been at least somewhat play-based as he doesn't know how to appropriately engage, and he definitely is starting to trust me more on the whole.

He still does not like when I'm in his stall, but is comfortable when I'm at the window or when he's in cross-ties. It's like his stall is his safe place.

I'm looking for advice about things to try and boundaries I shouldn't let him cross, etc. TIA!