I'm not the doctor, so keep checking back for his response. I cannot answer your question except to say that I cannot think of a good reason why removing a spermatocele should have any bearing on testosterone generation. I don't think he really has anything to worry about. As for the surgery, I can't tell you much. If you want, my web page has some links to other sites with information on spermatoceles.
Spermatoceles are common and increase with frequency with age.
Surgery is indicated if there is pain or if the spermatocele grows to a very large size. Removal of a spermatocele may cause obstruction of the epididymis which may affect fertility. In your husbands case, fertility is not an issue since he had a vasectomy several years prior. The surgery involves making an incision in the scrotum and delivering the affected testis through the incision. The tissue layer covering the testis and spermatocele is opened.