Can you suggest the best grammar book for my 12-year old son?

Dear Sir:

We came to States when my boy 3 and 1/2 year old. He is 7th grade at center talented school. He can speak good English and read a lots, most classes are excellent grades, except English- Grammar.

He thought his theory is right -English just write what does come out his mind, he dosen't like to follow the grammar rules, he got not good results. He tried to improve this subject, he ask me for help.

I eager to ask an expert like you to suggest me " How can I help my boy to improve and have confident for Grammar?"

I really apprecite your advice.

Have a wonderful day!

Best Wishes

Yang's Mom