My parents dog had a bump on his leg that started like this and over a week or 2 it grew to about an inch diameter before she took him to the vet. It ended up being a small benign tumor and the vet just had to remove it. It was not a huge deal.
I'm no expert on dogs, but I know a thing or 2 about people medicine which I imagine is at least similar to dog medicine. If it's not bothering him, then wait and watch it. If it does bother him, or if it keeps getting bigger and bigger, then have it looked at.