hate to be the one to tell you, but i wouldnt worry about necessarly treating with meds. First things first. You have to determine the source of the ICH. Maybe he had it before you even bought it, sometimes the dots are hard to see. also if in the begining stages, the little dots are sometimes not even present.
Then i would test your water and make sure all of the params are in check.
if these are all in check, I will explain to you what my method is for treating ich on clowns.
Get a good healthy flake food he will eat, then get selcon, Zoe and galic extract. Soak the food in all of this stuff for as long as possible, ie overnight if you can. Then feed often, not heavily, just often. 3 times a day if you can. Just make sure your not leaving excessive food in the tank.
Give the clown a good long month doing this, all along making sure that all the water params are in check and your system is stable.
This method has proven very sucessful for me in the past. ICH can be over come by a healthy fish's immune system. good luck!