Some men do report pains in their remaining testicle after surgery however this is not necessarily a normal thing and many times it ends up being nothing. I wouldn't be concerned that it is a complication from surgery or even a recurrence of the cancer. However, It probably is a good idea for him to get a scrotal ultrasound just to see what it going on for sure. If the pain is very intense then getting attention quickly is important just in case it is testicular torsion. Otherwise, just have him call the urologist and get a quick ultrasound.
hi there i had a testicle removed when i was 12, im 35 now and i too have dull pain after intercourse, but never after masturbation ask him if he has that problem sometimes if i have stronger ejaculations during intercourse that i find is the reason i have pain, dont know if this helps