Hi, thanks for asking!
The vet wasn't too concerned. She said because it seems to swell (more noticeable some days then others) it probably wasn't a cell mass. It seems to just be a more noticeable part of the testicle.
We did schedule his neutering though, in a couple weeks. If the vet removes it and feels it's abnormal he'll send it off for a biopsy.
He did get pre-surgery blood work done (because of past results) and his results were normal, which is always good to hear!
I'll update after his surgery in case anyone else has similar issues and are searching for answers.
