What other meds has your son tried? There are other biologics such as Remicade and Cimzia though I don't think Cimzia is approved in the UK although I could be totally wrong.
Has he tried any of the immunosuppressants like AZA or methotrexate?
I agree I would be discussing lack of improvement with the GI soon to see what else he may have up his sleeve.
My son went straight to Remicade upon dx, with a short stint of pred, while awaiting remicade approval and ped GI appt. He had no improvement with Pred and improvement right away with Remicade but when we got past the loading doses of Remicade he couldn't last the full 8 weeks without symptoms. His schedule was tweaked, his dose was increased and then we finally added Methotrexate. He was on both MTX and Remicade for 8 months then we dropped the MTX and now he only does Remicade, so far so good.
I do hope he is able to find a treatment to relieve his symptoms soon!