I'd save the picture and see a doctor. They might put him on antibiotics right away.
Apparently you have to wait 30 days before testing for Lyme to get an accurate test. Even then, the initial test they do can be wrong, so demand the better test.
Better safe than sorry! You want to catch Lyme before you get it!
Anyhow, that's what I've been told. We live in Virginia and the whole tick thing still freaks me out. We do NOT go to the doctor every time we get bit, or we'd be there all summer. (Seriously, I think I found 6 ticks on my girls over a two week period.) But if I had a rash like that and my kid rarely got bit by ticks, I'd be going to the doctor just in case.