If you can, and if you have time, you can make it up for him, or you can find a learning course online and let him follow. Online teaching is better than that of teachers. If the teacher teaches well, the children will certainly understand, and if they don't understand, they will definitely know nothing. Or you can find his classmates and make it up for him. If possible, you can spend some money outside to hire a tutor. Mainly to cultivate children's interest. If you are interested, it will be easy to learn.