My oldest son had mono his senior year in high school. He was gladly on total bedrest for 2 weeks. He even missed his senior ball. They were a rough two weeks, but then he recovered fairly quickly, although we made him rest plenty.
Yes, he will have mono in his system for life, but will never have a symptomatic flair up again. Most people had it when they were infants, and don't test positive, but can't get it again. It's a virus like chicken pox, so the worse that can happen in a flair up, is he can give it to someone else.
My best thoughts to you. He will be fine! Just give it some time.