[Sangetsu]

At his age he should be able to pick up Japanese quickly enough, I wouldn't worry too much about it. Much depends on where you will live in Japan, and whT kind of work you do. My wife is Japanese, but she prefers an international school where children are taught primarily in English, and then a day or two each week at a Japanese school so they can keep up with the Japanese language.

The larger cities have more options, in the countryside you will be more or less stuck with a public school.