Ok so uhm.. this guy keeps asking me out and I don't like him at all in fact i think he's a stalker but he won't leave me alone and he knows i have a bf soo what do i do??

Suggestion: Avoid.

Once he truly realizes you couldn't give more of a crap about him he'll decide to leave you absolutely alone.

" There's no point in asking a question if the other refuses to respond ". - Anndie Far

( Try to avoid conflicts. Do not warn your boyfriend.. this will only make it worse causing fraction of awkwardness. If you do this it'll be has if you were being the main bully rather than the stalker. I suggest you avoid. Have you had a chat with the stalker regarding him?)