Tea Tree Shampoo from Paul Mitchell!!!! Our family swears by this shampoo and we also buy the dog version for our dogs who get itchy skin during the winters! As with any shampoos though you are going to want to make sure he keeps his eyes closes so it does not get in them. Also, rinse with luke warm- body temp- water.
Also, make sure you are rinsing well and with clean water not bath water. Lots of times kids will also play in the "dirty" tub water after washing and get it all back on their scalps. This can cause the problems you are talking about.
Good luck with this, I know how frustrating it can be! Blessings to all!