I've read multiple places that he uses 8's, but haven't ever heard he uses 7's. Maybe that statement comes from the fact that his custom string set from Dunlop is available with a 7-gauge high E (7-9-11-20-30-38), though they're offered in heavier gauges, too.

From a Guitar World Q&A with the good Reverend a couple of years ago:

Q: Do you really use .008 gauge strings? If so, how do you keep them from flapping when detuning? And how do you get such a great tone, since I have always believed the bigger the strings, the better the tone?

B.G.: I, too, once believed in the heavier gauge string as a superior tone source. However, thanks to the graciousness of B.B. King I learned that a lighter gauge string offers superior playing comfort. Detuning requires some adjustment of attack, approach and feel. Try it. You may like it.