It is not a problem as long as it is not an undescended testicle. If that is the case he could be at risk for certain health issues.
A new sperm count could be taken by the health provider. However, I know an individual having a single testicle from birth, and I understand that one is all that is necessary. He was told by the school physician in grade school that one would grow and function as if two. In this case, he has lived a normal life, going to Viet Nam, getting married, buying a house, having a son, working a career, having $800,000.00+ in the bank, and so on. It should not be any problem for your future together, and best of luck to you both.
I'd say if he has always just had the one testical, that his sperm count is probably the same as it has always been. Unless something has happened to change that. Also has he had a sperm count done lately, or how does the doctor know?