I had it at 16. It almost killed me.
Let him rest. No physical activity.
I ate frozen grapes and jello for a while, I was so swollen.
He'll feel better if in the afternoons he gets up and showers, if he can.
No one else in my family got it. We were at our summer home and a string of cousins were visiting. Make sure you wash everything he touches and wash hands. It's spread by drinking after someone, kissing, through direct contact with saliva.
Change his sheets often, I remember being really sweaty and since I couldn't breathe well I drooled.
I had it in 1982. I still feel my lymph nodes swell up at times . I believe it is one of those that once you have it you can't get it again, but I did relapse during the school year.
I would rather have all four come down with strep (which they have) than one have mono.
Good luck. I was in bed for a good three weeks then really weak for another 8 months, my whole junior year.