My bf had the same thing happen the first time we fooled around, except he really didn't get an erection at all. He had been around the block many times back in the day, but it had been a long dry spell for him. He was very embarrassed, but I know that these things can be perpetuated if you make a big deal out of them. So I shrugged it off, told him it wasn't a big deal, and that I thoroughly enjoyed what we did do. The next time we got together, a few days later, the problem was solved.

So my advice would be to reassure him that it's no big deal, but don't go overboard with trying to comfort him or keeping that conversation going. Focus on telling him what you did enjoy.