What is a non verbal way of letting him know I'm not interested?

Just make sure you don't flirt with him at all. Don't be touchy feely, and maybe even casually bring up some guy you have a crush on, just to set in stone you don't have feelings for him. It may hurt his feelings a little bit, but it's better than leading him on now and telling him the truth later, after he's already fallen even harder. If you get him to realize that you do not like him before he says anything, you can avoid the awkwardness and feeling bad about denying him.

I think that you should either try to get a boyfriend or just tell him about some guy that you do like.

I'm not aware of any non-verbal things that won't seem distant and unfriendly. Only thing I can think of is to not follow him - while chit chating, pretend your phone is ringing and say, sorry, getting a call from this guy I'm totally into

Talk about this guy to your friend later ask him if he knows him - just make up an imaginary person. Describe him and make sure his description is the total opposite of what your guy friend looks like. And say how much your into that - like if your guy friend is tall, say the other guy is short - make it something your guy friend could never be.

Cheers