The mother is being both cruel and very very stupid in a number of ways, but you must look on the positive side which is that this situation *will* change. The little boy (he sounds young) will decide long before any 'legal' age that he wants to see his dad, and if he can he will jump on a bus or whatever and do it ! So take heart from that.
You will already know that your son can get full and enforceable rights to see his son, the problem is if his ex decides not to play ball - but she will eventually, the little boy himself won't stand for it in the long term.
Good luck, but there is light at the end of the tunnel.