Beware, Aries28, there’s a good chance he’s up to something.
My ex-girlfriend (now good friend) has a father who is a complete jackass. When she, at the age of 18, was pregnant (not by me - not my gf at the time), she needed to apply for government assisted health insurance. Her application came back denied, due to a large amount of money she supposedly had stashed in various bank accounts.
Turns out her jackass father had been stashing money away in her name, in order to avoid paying taxes on it and to keep his child support payments low. She decided to withdraw the least expensive account, and see what her father had to say about that. Sure enough, the rest of the money disappeared shortly after that, and she started getting very nasty phone calls.
Fast forward a few years. Her jackass father is asking her for her son’s SSN, so that he can be put into his great grandmother’s will. Wisely, she is refusing.
I encourage you to start checking into what your ex is doing with your son’s SSN as soon as possible.