We can "only guess"... but I doubt it is a tumor given the dog's young age.
You say "red" does that mean the hair has fallen out? Does the dog react to your touching, pain?
From what you say he couldn't reach it with his mouth, I assume. Could it be a bite from a spider?
I'd watch for a couple of days and if no symptoms develop I'd forget about it, if not then a visit to the vet would be my next step... maybe a good time for a general check-up, make double use out of the $$ investment.
Well it's in a spot where he's never had much fur. His coat is really short so behind his ears and where his legs meet his chest, his skin is pretty visible. He doesn't react to it at all. I squeezed it a little to see if it was hard or not and he just happily laid there and didn't move. And I suppose a spider bite is possible, but it's not too likely since it's been so cold here I haven't taken him outside too much.