I had this problem last year, except from, I'm ashamed to say, I was the one acting like I hated the boy when around friends.
You want to know why? Because all our friends could see that we liked each other, and would tease me constantly, so I pretended that I hated him to get them off my back.
In the end though we got together, which I'm glad happened, even though we aren't together anymore.
So I suggest you ask him what the problem is, it can't hurt can it?
I hope this has helped you :)

Try confronting him when you are alone, like just simply asking him if his friends have a problem with you. If he says no, then ask him if there's a problem with being near you in public. I had this problem a few years ago, and this definitely helped me. Try it out, and if he has a problem with you, drop him like he's hot. If not, then you'll both be fine and you could even mention that he's been acting stiff around you in front of others. Hope this helps!
~Hannah~