I had a mare with qld itch once and we used an oil or something that we got from saddleworld. The oil was a special mix for qld itch that relieves itching and hepls get rid of the itch. And you don't have to wash it out.
There are several other remidies that don't require washing out as well so you could consider them.
But if you do want to continue using what you have it should be fine as long as you make sure he is well dried.