You must mention which disease he suffered,otherwise it is difficult to give answers.
In general case u must consult a psychologist, by which he can receive counselling sessions, where he can motivated to appear in board exams.
Family support is very much needed.
If ccondition get worse consult psychiatrist.
Health Tips
1.Dont discuss too much about him in home.
2 dont disscuss about him with neighborhood, relatives and other persons.
3.dont scold him ,love him and try to communicate with.
4.He can appear in board exam next year also so dont push him or pressurerized him.