Social security to be a beneficiary?

My sons great uncle on his fathers side would like my son to be a beneficiary. But he said he needs his social security number. Me and my sons father are not married and I don't get along well with some of his family. I have no problems with my sons great uncle but it is my sons grandmother asking for the number. Me and her have problems. Is it necessary for them to have my sons number? I really don't like giving that kind of information out. Also is there a website i could visit that will explain to me exactly what a beneficiary is and why is social security is needed. Ive never had this happen on my side.