If he is good at other activities then he must be a brilliant child. Dnt make entducation a routine and boring activity and pls dont compaire him with othet children.May be he has missed out some basics becoz of which he is not able to cope up now or the method of teaching is not intresting to him.Introduce topics as activities (we have plenty in this website) and take each one step by step.Even if u have to teach him Kindergarten concepts thats fine. Dont make it a long study hour kind of thing just keep it short intermixing with his favourite activities. Introduce words games like scrabble and memory matching games.
Most important of all have patience. Good Luck !