You can't put him on if he's not there and you're not married unless he completes a special form (which has to be witnessed) and you take that with you. Otherwise people would be putting anyone they felt like down as a father and the men wouldn't have a clue! Enough loonies out there stalking celebrities etc without giving the the means to name men as fathers without any say in it!
Not being on the certificate makes no difference to maintenance. Explaining that to him might just make him change his mind. The CSA will ask you who the father is, you tell them, they contact him. If he denies it then he has to agree to a panternity test to determine fatherhood.
Being on the certificate might make a real difference to your child's feelings of who they are and where they come from. I felt this was important and ensured me ds's father was on his certificate and got his family on side who convinced him to do it for the child's sake.
Being on the certificate has implications for him too. It means he automatically gets PR. However he can still apply to court for this at a later date even if he's not on the certificate. He can also apply to be entered on the certificate if you haven't included him and he wanted to be.
PR basically means that he is recognised as a parent with responsibilities to his child. It doesn't give him any rights, but it does give him responsibilities to ensure the welfare of his child and he is then able to get details of school, medical records and have a say in important decisions. Your child has a right to a relationship with both parents. Your ex has a responsibilty to his child which he may or may not accept.
One other point that someone mentioned in a previous thread on this topic. If you have a new relationship in the future and you and your new partner want your new partner to adopt your child this is very much harder with a father named on the certificate as he has to agree to it.
Your child's birth certificate is a legal document that will be with them through life. IMO it's very important to include both parents on it and for a child to know from the outset that they have two parents who both acknowledge them as their child.
If you ex is reluctant I would speak with his mother and see if you can persuade her to discuss it with him. If he won't/can't go to the registry office with you get one of the forms he needs to complete sent to you and make sure the registra knows you are waiting for this as they won't keep reminding you to come in if they know you are waiting on the bit of paper.
best wishes
Gilly