Assuming you know the basics about OCD, how it gets worse with stress, and is a sort of coping mechanism which makes some people feel safe, etc.
One thing is almost certain - you can't force him out of it. It's almost impossible. It annoys me when my father tries it with my mother.
If it's possible then maybe getting him to talk about his fears with you (pretty deep i know), or talk about what's stressing him and see where it goes. Maybe find out when/why/how it all started. Though it might take a while for him to really open up about it if he's embarrassed about it.
He may have to see a therapist if his OCD is something that bothers him and he wants to change it, because they will be better able to understand it. But you as his girlfriend are better able to understand him, and help him that way.
Hope it helps :-) Good Luck!
J x