Just basing this on my experiences. I don't know all the details of your relationship with him and his friends. So don't take this as like 100% the thing to do.

As someone who just doesn't have romantic interest with anyone. I sometimes don't realize if I'm being flirty or "leading someone on" when I'm just trying to be a nice friend. So i've had this situation a few times before where I'm just being friendly but it's taken as interested in a "more than friends" relationship way to the other person. It can be kinda awkward explaining it but it's best to make your intentions clear otherwise they'll just continue unwanted advances if you don't tell them.

If you've told him you have no intention of your relationship with him being anything more than platonic, that his actions are making you uncomfortable, and he hasn't stopped I'd say give him a final warning. Generally if they don't stop after you've told them before, then they probably won't and it's not worth being friends with someone if they make you uncomfortable.