Your rats around closing on 2 years old now?
When I used to do breeding its right around this age that they start to get Tumors. If I still breed I would have tried and line breed them for more reisstance. It seems like they are very Tumor prone. It is also one of the reason why rats of often use in medical testing.
Hope your boy is ok
neutering/spraying them is the best prevention. Unfornately the procedure is so expensive compare to their relative lifespan...