When rinsing you want to get a chill to him, so you want to rinse for at least 15 minutes with cool (cold if you have it) and then blow him out and get him in the shade with fans if you have them. The theory is if you put him away wet, the body heat will heat the water on him and they are actually hotter when left wet then when dry. If he is stalled with a fan blowing on him you can put him with a little moisture on him the fans will cause it to evaporate and cool him some.
The rinsing helps but the combing and brushing is what will stimulate the hair to grow. I would rinse him and then take a rice root brush to him while wet, with the fan blowing on him from behind to head, and brush him forward at a 45* until he starts to dry, then blow the hair at the same angle. If he has wild spots where the hair wants to go its own way, when he is wet is the time to start getting it to go the right way. Brush everything forward, then go back to the spots that do not want to cooperate, and brush down then up, sometimes down and back then up at the 45. It takes a while to get them worked out but if you work at them you can get it trained. The old story goes "When using the rice root brush, use your right hand on the left side and brush until your arm feels like it is going to fall off, now go to right side of the animal and use your left hand until your arm feels like it is going to fall off, now start over".
If you can get the hair to go the right direction and get it trained, it will definitely help give him a younger fresh appearance. Calves with trained hair even if they don't have a lot of it will have a softer, youthful look to them.
Just curious, have you figured out your protein and fat percentages with your feed and additives, he should be getting about 14% protein and 4.5% fat. I really like what the Stabilized Rice Bran does for their cover, as well as the hair, you get your hands on one finished with the SRB and the finish is smooth and even over the ribs.
I think if you can get your calf finished, some belly to him, and get the hair trained he will look the part.