Eventually we noticed that he had some problem with his testicle. He was about 8 months old and had never had a problem before, but now his testicle seemed swollen. He did not seem to be in pain but his interest in exercise was not normal, so we wanted to do something. So far as we know he was never injured. We took him to the vet and she said that she could neuter him and that might solve the issue, so we arranged the surgery.
She knocked him out and neutered him. He got two stitches and seemed not to mind. He was sleepy the rest of that day but walked around normally later in the day. We took away his wheel so that he would take it easy. He seems to be recovering nicely, and hopefully will start playing more soon. He does not seem to be interested in his stitches, which is good because we worried that he would bite at them.