Hi,
Welcome to the ehealthforum and I am glad to help you out.
What your boyfriend seems to be having is retractile testicle. A retractile testicle is a testicle that may move back and forth between the scrotum and the groin. Sometimes, the retractile testicle is no longer movable and instead stays up in the groin. When this happens, the testicle is no longer considered a retractile testicle. This condition is called an ascending testicle.
Your boyfriend should get it evaluated from a surgeon and get an ultrasound done to confirm the diagnosis.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care.