Are you registered as being the father of your son? If so, call Social Services and have them intervene and you probably will have to go to family court in
order to get visitation rights or even joint custody rights established.
In the meantime, can't you find out where your ex lives? Your son must
feel very abandoned by you (although it's not your fault), he has no one to
fend for him, so the sooner you act the better. How old is your son?