Donna take Winkie to the vet first and foremost. Has he been neutered? I don't know what your vet will suggest. Depends if he finds more lumps elsewhere I guess or if they are contained within the testi. Cancer is just huge in rats. The rate is astronomical. If I am not mistaken it is in the 70-80 percentile maybe more. I say get him checked and get his checked quick.

Prayers for Winkie and let us know what the vet says.
((((hugs to you and winkie))))
Michelle