It would be a good idea to contact the school and organise a meeting between you, your son and teaching staff to discuss his options.
Child benefit usually ends in the first week of September for those children who leave education before the summer if they haven't entered full time employment. So, you shouldn't have to repay anything at this point.
If son decides to go on to college rather than school CB is payable until the 'child' reaches 19, or in some cases 20 years of age.