You can put any name you want on the birth certificate(as far as naming a father).This is why so many celebrities get b.S. Paternity suits.
This may sound like a nice idea now,but if you ever need to take the real father to court,it could complicate matters.If your friend wants to play a father figure,that is sweet of him.Let him have an active role in your childs life.The birth certificate is a legal document and you should provide the most accurate information you can,just in case of legal problems down the road.Good luck to you.
Patty