What is it?

it's normal puberty stage stuff... he's probably scared that something is wrong with him because he is going through changes in physical and mental ways, have your husband sit down and try to talk to him about this. he is more likely to open up to a male than a female right now

I know boys talk better when they are doing an activity. It's a guy thing. Definitely as they get to preteen, many of them start to clam up. Your have to be available for them to talk when they want to (often late at night) and fine tune your powers of observation and networking with other parents. It's really important for your husband to be the primary informant and confidant about what's going on in his body and mind. I make no bones about checking in on texts, facebook etc. and taking note of their friends and activities. With one of my children and her friends, we as parents make a pact to always be aware of what was going on and to alert the others if there was any concern. He may not seem like he is listening but keep repeating what you expect of him, what the rules are and what behavior is appropriate. He hears you.