It is normal for everything to be swollen for a while after the birth in both girls and boys.

My DS had a hydrocele (fluid retention on scrotum) for a few months. My Doctor said it wouldn't cause pain, and should subside within a few months. Well, it went away by about 10 weeks from memory.

Your GP will check his scrotum as part of the 6 week check, I'm sure he's fine!