I always suspect my 8-year-old son has ADHD. He is very forgetful, finds it difficult to take instructions, and has trouble concentrating. We just moved to Hong Kong three weeks ago, and he has been very grumpy and unhappy at home. His teacher says he is an angel and there isn't a problem. I took him to the hospital for a simple evaluation, and the doctor said he doesn't have ADHD.
There are many things besides ADHD that cause problems in children. A big move is always very stressful for families and especially for children, so you might want to see if your son remains grumpy and unhappy after a few more weeks have passed.
You seem to have already done some research with your child's teacher and discovered that your son shows no difficulties at school. I would continue to keep in touch with your son's teacher at regular intervals to make sure things continue to go smoothly for him in the classroom. If you still feel there is a problem, locate a pediatrician or family doctor so that your son has someone to see for regular check-ups and routine medical concerns. When you meet with the doctor, explain what you have observed and ask if there is a mental health professional who can perform a comprehensive evaluation of your son and make recommendations. The problem may not be ADHD, but getting to the bottom of the problem would be best.