The best substitute I have found is Adderall XR. You can take the capsule apart, put the beads in a spoonful of yogurt, puddding, etc. They can't taste it, but you have to make sure they don't chew before swallowing because it would break open the extended-release capsules. Also, in order to get ready for swallowing pills, use plain M&M's for practice. You can even start with the mini's to get him used to it. That way if the M&M melts a little, it won't taste bad.
Good luck!!
txmomof3