Dont worry too much.. There are many many children who do not pass their exams.. Parents often feel they lack concentration, no interest to study, or he is very careless about his future etc.....
But, thats not always right.. First of all, sit down with ur son and find out wats troubling him.. If he lacks interest, if he is feeling inferior to his classmates or whatever is the problem exactly.. No need of any doctors..
Also, please do not scold him or be harsh on him.. am sure even he will be sad about his results..... Instead, try to encourage him more and more and appreciate him for even a single correct answer he gives.. try and boost up his confidence level.. If parents can support him to the best, then definitely the child feels secure and cared...... He also will try to make his parents happy.....
Also, if the child is finding subjects very tough, u can try changing his combinations.. Never mind if he loses 1 year.. but ur son will improve a lot in that year which is more important..
Hope i helped.. and all the best for ur son