My son has a 3 month old daughter but she registered her without my sons name on should she have put known father on it or doesn't it matter

If yr son gets a dna test and proves he is the child's dad he can go to court and apply for supervised visits to start with then get shared custody as time goes on .

Please pay more attention to posting guidelines. There is a Read First sticky in the Children forum (where this was moved from) that directs questions of a legal nature to this forum.

If your son was not put on the birth certificate, then he has no legal standing for the child. If he wants to be the legal father, he will need to take some action depending on what state the child was born in.