Ring his secretary and get her to book a follow up appointment for your son in clinic to be seen by the Consultant personally. If that can't be arranged for whatever reason, write to the Consultant. Dont bother ringing again. If he's not that way inclined to ring patients back (and most of them aren't) you are wasting your time. Have you thought about requesting a second opinion? Your son's condition is obviously giving the Consultant concern that he has discussed it with his colleagues - perhaps he's not keen to undertake surgery himself and may welcome your suggestion of a referral elsewhere? All that said, you need to know what's happening.
If all else fails, you should write to the Chief Executive of the hospital trust who will take up your complaint with the Consultant and he will have to respond. Hope it all works out for you and your son.