Ok get your notebook out lol, I just went through this with my calves. Your calf is shedding, it is what his body is telling him to do because it is starting to get warmer. The first thing you need to do is comb, comb, and do some more combing. Use a curry comb, scrub brush, show comb, anything that will take the hair out that wants to come out. He will keep shedding until you get all his old hair out. So keep combing on him. It took me around a week to get all my heifers dead hair all out.
Once you get all the old hair out then the new stuff can start coming in. It will take some time, but with washing, combing, and a somewhat cool environment, his hair will start growing back. I recommend using a roto brush from sullivans, it is the same as the rice brush only bigger and does a better job. Working the hair is key to getting it to grow.
Hope this helps.