From what I can gather, this lad rather naively 'wanted to help the unfortunate' in Syria, converted to Islam in an idealistic fashion and dropped out of his studies. I'm not certain whether that constitutes terrorism, or even if he has actually broken UK law. He sent an SOS to his parents requesting a money transfer, and after they complied, they were interrogated by Police here for 'helping a terrorist'.

Now I'm not a parent but I imagine anyone's mum and dad would be concerned by their son's need for funds and would try to help him.
His father is 'an organic farmer', in no way connected to Islamic factions.

I would hesitate to bar the lad from coming home until further information is obtained. In any case, is there actually a Law, enacted by Parliament, that expressly forbids a UK citizen from re-entering the country? If so, under what specific conditions?