Do you have your son on iodine? Stephanie Buist, Iodine advocate [http://www.naturalthyroidchoices.com] has a son who's ADD and might be able to offer some suggestions. When her kids were small, she crushed up an iodoral tablet and put it in applesauce for them to take. Can you get him to eat salsa with cilantro in it to help pull out heavy metals?
I think Clarkia is an EXCELLENT idea for your little guy. You can hide the taste in strong juices like cranberry or pomegranet. The dose is 1 drop for every 10 lbs so you won't need much for him and it can be masked really well in a couple of ounces of juice.