Be direct. Ask him what's wrong. Tell him you can see he's acting weird, and that he should just tell you what's going on rather than letting this drag on.
Don't say all that in a whinny way, be frank and straightforward.
It sounds like he's upset with you - maybe you said or did something that unintentionally offended him without you realising. That'd make sense why he's having a hard time opening up.
However, he is also being pretty childish by playing the silent game.
If he doesn't spill the beans after you ask him what's wrong, then you should just move on. It could be that he's attention seeking - in which case you just need to move away from the mind games and hang out with people that don't ignore their friends!