Personally, I would just be up front and honest if I was in the same situation, and have been a couple of times. That said, if you're both just that busy that you can't find time to go out, it seems like it's not that great of a fit, at least for right now.

Maybe explain that you like that he's able to keep himself busy, but you feel like conversation and interest is dwindling and would like to know if it's just a simple case of "too busy" or if he's not into you. If he does say he's still into you but just too busy, I would tell him straight forward that he sounds like he's too busy to date right now and that you'd like to talk to him again when he has more free time (if that is what you want, of course).