Enrich his learning at home with books (everybody in the family should read at the same time), travel, outings, academic camps, building projects -- any kind of activity where he can use numbers, map-reading skills, etc. There are usually learning centers in larger cities that are a great place for remediating basic skills.
This is not an overnight solution, but by constantly involving your son in the excitement of learning, especially in everyday real experiences, he will learn those things he needs to know to be self-sufficient and happy.