I had mono when I was 18 and in college. I didn't get it diagnosed for probably 6 weeks and then continued to be sick for probably 6 more weeks (of being EXTREMELY tired - barely able to make it up stairs). My grades suffered terribly and it took the remaining years in college to pull them up. Basically, I had the energy to attend classes, but no energy to do homework or study.
The best advice I can give you is to keep him home at least a week and let him sleep non-stop. I think if I had been able to stop and rest, I might have recovered more readily. But, as it was, I could not stop and just kept trudging through it - which didn't help with recovery.
The other problem with mono is that you are susceptible to a reoccurence once you have had it. You need to not over do it.
Also, at the time that I was sick, my side hurt and the doctor told me that my spleen had all kinds of toxins in it and was working hard to clean out my system - it was my spleen that hurt. So, even though I started feeling better, she said to take it easy because if I was over active, I could rupture my spleen.
Good luck!
L.