Yes, you need legal advice. There was someone else on here who just got a good solicitor I think for very similar issues. I'll see if I can find the thread and post a link here later unless someone beats me to it.
That said, you should know that once you and your son are in the US (assuming that is how things end up), you can pursue him for child support via international legal channels. From what I understand, the UK is actually better about enforcing international child support orders than they are about their own ones via the CSA!