1. Seat CushionA semi-inflated rubber cushion with bumps for lots of sensory input can literally give your child some wiggle room -- he can get the feeling of movement without making too much of it. The circular Disc O' Sit, in 12" or 14" diameters, is one to try. You want something that your child can get up on and off of comfortably and with a minimum of fuss, so that big inflated cushion with a hole in the middle that you got for someone's hernia is not the way to go. Work with your child's occupational therapist to find something appropriate, and make sure to write your child's name on it so it doesn't go astray. When your child outgrows it, ask the teacher or therapist to pass it on to another child in need.
3. FootrestOften, kids whose feet don't reach the floor feel unsettled and unbalanced, and are more likely to kick, fidget, rock and roll in their seats as a result. You don't need to buy a fancy footrest; just place a brick, block, box or other hard heavy item beneath your child's feet so that his feet rest firmly on it and his legs are bent at a comfortable angle. Make sure the item's heavy enough that he won't be pushing or kicking it around. A pile of books might seem like a good emergency option, but they'll likely get slid and kicked around in a most distracting way. A single solid heavy object is the way to go.
4. Separate Desk and Chair
All-in-one desk-and-chair combos are dangerous for active kids, since rocking the chair means rocking the whole desk and sometimes knocking things off it. Your child may be more comfortable in a desk with a separate chair; if the teacher can find one, it may make a real difference in classroom decorum. Slipping a cut-open tennis ball at the end of each chair leg takes the noise out of any scooching and sliding that does take place. If this turns out to be a good option for your child, ask that it be added to his IEP for future school years -- even if you have to do it in the parent input statement.