I really don't know how to begin to ask him.
yea I'm absolutely rubbish at confronting people or having "heart to heart's"! I always bodge them!
My advice would be to try and keep it as light and breezy and "unheavy" as possible....just take it in tiny baby steps. Maybe, at first, don't even mention any problems, just try talking to him more again, asking him how is he etc, and see how he responds. And if he continues to snub you, then it might be more obvious by this stage and therefore easier to ask him if you've done anything to upset him?
just a thought, but is it possible he thinks
you've stopped liking
him, and so is just responding in kind? I only say that cause I've experienced that before, being a social phobic and all!