It's the lack of routine and structure that he receives in school. Maybe do a homeschooling thing during the summer with him. You can find all kinds of neat videos on YouTube with songs to teach ABC's, counting to 100, the days of the week and months of the year - all things he would learn in public school and his program. Set it up so it is structured like school - he needs that and thrives on it. It's why he's becoming more aggressive, he knows he's off, but doesn't know how to stop himself because he isn't used to the freedom. Even Elizabeth at 17 years old, isn't a huge fan of summer, because she likes the routine, the challenges of her studies and the structure she has at school.
The neat thing about doing this, is that his little brother could dance with him during the videos and learn as well. I did homeschooling for five years with our girls and my youngest is five years younger than her sister. A lot of the videos are upbeat and the boys can dance and jump around - it's exercise AND they learn at the same time. I hope this helps some.