If his mother will waive the exemption allowing you to claim your son then you can do that. You can print the waiver by using the hyperlink here: Form
8332 She can complete it and sign it for your tax file.
You will indicate that your son doesn't live with you since you are the noncustodial parent and then answer the questions about the waiver.
You are not required to report your son's SSI as income on your tax return.