Make him finish what he starts but don't force him to start something he doesn't want to do. Learning to count the cost and count it correctly is one of the great lessons athletics teaches us. Maybe, he has counted the cost and decided it doesn't add up or maybe he has miscounted and will later decide that he misses it more than he thought. If so, I would still make him sit out the rest of the season. He could start again during freestyle or wait until next year. Missing one year will not determine whether he will be successful down the road. In fact, it may help him dedicate himself more to the sport if he decides he really misses it. If he doesn't really miss it, than it was probably more your dream than his.