Help with kid hitting uncle

Hey so this might be a silly question. But my 5 year old nephew sometimes when I'm with him just comes up to me and hits me.

It's fortunately rare, but I really don't like it. The first time we were playing video games, me and his siblings (3 and 7), and it was calm. Then he out of nowhere slapped me open palm in the face really hard. I was firm, I told him strongly: "do not hit me like that and it is not okay to do that". He then retreated and seemed shy about it and I moved on and didn't linger on it. I assumed it was some kind of overexcitement impulse beyond his control.

Quite a long time has passed and I don't spend every day with him. And most times I see him it's a brief family visit or park trip. But recently when I spent a lot more time with him on an extended trip (kind of like the last time), after a long day out and play we got back to this hotel where I was tired and sat down. He was roaming around, the kids weren't playing. And I guess he wanted to initiate play with me? But he quietly walked behind me, climbed a chair and hit me hard in the back of the head with a closed fist.

I was so tired, I just said "no" to him and not much more and as a couple minutes passed, the pain got quite bad and I said "that really stung" to him and his mother and she very playfully and hugged him and said "don't hit people" to him. And he kept playing with his Switch not taking it in. I'm worried I wasn't nearly strong enough on it, and his mother was too light about it.

I'm probably overreacting but I'm concerned each time he's hit me it's been out of nowhere, I've been at rest and I have started flinching if he moves erratically near me. I'm quite unnerved by it. I don't mind rough house type play, and I'm very careful to be clear about starting that play and ending it and when I do I carefully detail not to hit me in the head or private parts or to hit too hard. And I'm very careful about winding down after and I've never had a random hitting incident when we've played like that. I don't think he's bad, but I think he has stimulis issues - maybe adhd or autism maybe. I'm no expert, he just flits constantly like a puppy and bangs his head into any sofa or cushion he can and constantly runs.

His mum gets hit sometimes but I never see it. I've never seen him hit anyone else, certainly never his grandparents which makes me dislike this treatment even more. He knows he can hit some and not others.

I'd like to say I'm the totally calm enlightened person and I practically am, I abhor violence and avoid it like the plague but when I get hit in the head I have to manage anger like anyone else. And I don't want to frighten him, but I also don't want to tolerate anyone hitting me in the head ever.

I'd like some advice on how to parent as an uncle for these things. I don't have kids, and I don't want to be treated this way. I'm surprised it's bothered me so much. His sisters don't hit me at all, and I know he loves my company - I get on well with all of them. Maybe I'm being oversensitive.

I'd appreciate any advice you can give.