Hi, I'm actually up in Chiang mai but am in a similar situation in that my ten year old nephew has just come to live with us.
I honestly think the best way to teach em English is to just do it yourself. The school he's attending is worse than useless in terms of the education he's getting but his progress has been remarkeable in the 4 months he's been living with us.
My wife spends about an hour helping him with vocabulary every evening and he can't wait to practice with me as soon as I get home.
I hope it all works out for you.