I had it a year ago this last September, it f*cking SUCKS.
The number one symptom is insane fatigue. All you want to do is sleep all day, every day. You don't want to move, you don't want to do anything. Your throat is in raw pain almost every day and you are just dragging ass and generally feel like sh*t. When you DO sleep it's very active- wake up a lot, drenched in sweat, that sort of thing. It is a zero-activity viral infection. From the time span I was feeling my first symptoms to the time I was back to doing my full workouts in the gym was about a month and a half. I think 3 months is a stretch- from what my doctors told me at the time it doesn't take that long.
Nothing he can do. Rest, fluids, sleep. Over the counter drugs to handle the fever and pain. That's it.
The good news is that he'll never have it again. After you have it once, you're immune to it and all of its relatives (which if he's REALLY lucky he only caught a close relative on MONO and not the worst strain) for the rest of your life.
I must have had a really weak strain or just a really good immune system (I only had chicken pox for 5 days), but I was only sick for about a week, and I really didn't have terrible fatigue. The biggest pain in the @ss was waking up every two hours drenched in sweat. Needless to say, I had to change several pillows, sheets, mattresses, sleeping bags, etc., every night. There is nothing worse than waking up completely drenched with sweat completely saturating all your bedding. I had a rotation of pillows and sleeping bags I slept in for about 5 nights.