I am divorced and I don't have my son's social security number. His mom won't give it to me how can I get his SS#

With the proper documentation you should be able to get it from the Bureau of Vital Statistics.

If you are named on the child's birth certificate, you can obtain a copy from the county the child was born in. Take that to your local Social Security Administration office and request a card for your child.

Or you could file a complaint with the court of jurisdiction. She seems to be being petty and judges are not fond of that.