Step one is the haircut every week.  Get him used to your hand on the back of his head while bent over shirtless in the bathroom.

Step two, catch him as he is going into the shower and suggest that just prior to showering is best for a haircut, so he can wash off the stray hairs.  Soon he is used to bending over in the bathroom while naked. 

Step three, get him drunk, take him to the bathroom.  Let the muscle memory kick in.  Once he is bent over naked, invite the rest of the platoon in to use him.