100% this:
http://www.kaizenfitnessusa.com/weekend
Sunday classes @ TI, 2pm
My sixth grader started beginner on Sundays last fall a year ago with the little kids (like 5/6/7 year olds.) I was sure she wouldn't go back the following week. I emailed them and they switched her to intermediate, she was not the only white belt and it was a very positive experience.
I suggest signing him up for intermediate where there will be older kids. He is not likely to the be the only 6th grader, no experience, white belt. After the fall session, there were plenty of older kids without experience that started in intermediate. I was relieved, my kid is now a yellow belt and I am hoping she continues to black.