Rat Testicles?

Question
Hi, I have a full grown male fancy rat. Getting close to a year old.
I haven't had him for very long, but in the first couple of weeks, I notices that he had a large tumor-like abscess in the left side of his abdomen. At first we were joking around, saying it was a lopsided pot belly, but when it didn't go away, I got nervous. I also noticed that he only has one testicle, on the corresponding right side. He acts absolutely normal, but it has never gone away. I know that when they get scared, they can do this, but it has gone unchanged for many weeks.
If you could help me, that'd be great.

Answer
This is something that a vet needs to feel with his hands. There isn't much I can do for you here except let you know that.

Rats can have retained testicles and they can cause some issues. It is fairly important that you have him looked at.

Hope he does okay.