Still only one testicle at 5 months old...?

My son is 5 months old and he still only has one testicle. I was wondering if that was hereditary? His father has both but some people in my family and his had problems with that too. Also, I know a father and son who both only had one testicle. I can't find information on it anywhere. Please help thanks. When should I expect him to get the other?