Perhaps he has a negative conception of commitment, even if he has no intention of straying. There is a lot of pressure on guys in their 20s to not commit to anything, be it a relationship, career, whatever. It's a favourite subject of sociologists these days.

Even if your boyfriend doesn't overtly act out, he's probably absorbed a lot of ideas about how guys "should" behave...in fact, if he watches a lot of sports, that's practically a guarantee, given the advertising beer and car companies use these days.

I would advise trying to get him to open up in as low-pressure a way as possible. It doesn't sound like he's at risk to cheat, so take things slow and try to understand why he's so non-committal.