A.D.
Molly,
How did the doc appointment go?
I know this post is a bit late, but if the lump just came up quickly, chances are that it could be a lymph node from an infection he is fighting, even if he has no other symptoms. It will likely go away on it's own. Getting it checked out is important though, so good choice!
I had a VERY lumpy head as a baby due to my lymph nodes. My poor mom was always worried. On the head They tend to run across the back of the head to the neck, behind the ears, in front of the ears, but they can "chain" in other places too. If you feel my head now, you'd still notice a bunch of small lumps (after nodes swell several times, they can stay slightly enlarged).
Let us know how the appointment goes.