Many 3 year olds don't sit still, throw tantrums, don't listen, and are easily distracted. How you deal with it is important. Montessori classes are wonderful for kids because they are kind of unstructured and kids get to choose what they want to do. They may not be the best place for children who might need more structure.
I would suggest that you "obtain a copy of Lynn Clarks' book SOS: Help for Parents". If that doesn't help, then you might want to take a look at, "The ADD/ ADhD Answer book." , by Susan Ashley.
And yes, if the only language (or the majority) he was getting at home was Japanese, then his English skills will be delayed. They will catch up. And it could account for some frustrations at school, but shouldn't at home. So you might take a look at both books. Good luck