I just got a new rat on Friday and I have just had a good look at him (as he was very scared I couldn't before.) I have just seen a lump on his tummy. He is a month and a half old. Could it be a tumor at this young age?
I kinda doubt it's a tumor at his young age, but anything is possible. It you just got him, I would be more inclined to think it's an abscess, maybe from a bite from a cage mate? Rats heal so quickly, that a small infection might have gotten sealed up in there. Definitely take him to the vet. He needs to go for a checkup anyway since you just got him.