Most Helpful Guy
I know it's a bit extreme, but you could always create a fake Facebook account or something like that of a girl and try to 'chat him up' with that account and see if he 'goes for it'? Of course it's very wrong that you should feel you need to take these kinds of measures, but sometimes the end does justify the means.
What you are describing does sound suspicious, but then again it could all be circumstantial and you're making more of this than what it really is. The truth is that you don't really know, but as his behavior is becoming increasingly odd, your suspicions grow. Somehow you need to find a way of finding out for sure what's going on, although exactly how you go about that I'm not too sure.
It's a horrible situation that you're in and I really hope that your suspicions are unfounded, but it's best to know the truth and to go on the way you are. You could either confront him completely about everything that you have on your mind, or try to catch him out, but either way you can't go on wondering, worrying and stressing about this... You need to find a solution!
I wish you all the best with this and feel free to ask me anything if you feel I can help... :o)