Why do you want his name added to the mortgage? I'm not sure that'd always be the wisest decision. If his name is already on the deed, he's fine as far as ownership is concerned. And whether he's on the mortgage or not, he can pay (or help you pay) the mortgage. Adding his name might just expose him to additional liability.
To address your specific question, you can check with your lender. But, really, check with a real estate lawyer before doing it. There are a lot of drawbacks and not many advantages to doing so.
Hope that helps.