Children with ADHD need to have structure and predictability in their life. They need to have a set time when they work on homework each day and a neat, organized place to work. There needs to be very little to distract them nearby (such as television or cell phone). They need to be allowed to wiggle while they work and take movement breaks away from their workspace very often.
I think I would meet with his teacher right away to try to determine what he is behind on (or IF he is behind). Then each day, he could spend a little time on those subjects or concepts. If he has been an "A" student up until now, the chances are that all the children are behind on the same things since the curriculum changed. It is possible that his teacher is already working on those things.