How to tell if he's sterile?

My boyfriend and I use the withdrawal method because i'm allergic to latex. We have been doing so for over 6 years. I think we are just brilliant at being accurate but someone has told him he might be sterile. he was born premature and needed surgery for an undescended teste. this increases the risk of many things. I hope and pray he isn't sterile and he's too proud to go to the doctor to find out. I want kid(s) with him, no one else... How can you tell without medical help and don't say "try not withdrawing" because we can't afford a baby right now. I just want to know so we can look into working round it if he is sterile. thanks.