Hello Susan, I'm Dr. Deb.
I recently came online and see that your question about your dog hasn't been answered. I'm so sorry that you've had to wait for a response,but if you still need assistance, I'd like to help if I can.
It's certainly possible that if he experienced trauma of some sort (given how clumsy pups at this age can be) and bumped his head on something. If that's the case, then a seroma (serum filled space) or hematoma (blood filled space) might have developed.
Without aspirating the swelling though, it will be difficult to know for sure.
These swellings aren't painful and the fluid inside them will eventually be resorbed by the body over several days time.
Another possibility is an abscess or infection. These swellings are usually a little painful and uncomfortable when palpated; there's also usually evidence of a bite wound or some other sort of wound.
If your pup is otherwise acting fine, then I might monitor the swelling for another day; however, if the bump is still present and not showing signs of regressing (or if it's enlarging), then a vet visit may be prudent.
I hope this helps although, again, my apologies for the delayed reply. Deb