It sounds like a 'commitment' issue - a lot of guys are scared at the thought of srttling down with one person - even though it could be the very best thing for them -and as a married man with twenty-five years of experience in that area, I know of what i speak!
It is certainly worth trying to talk it over - if he won;t talk to you, then he's not worth fighting for. At least you may get a good reason, although I doubt it, since probably even his is not sure why he is leaving you, but have a go.