Recovery is extremely variable, as the other posters mentioned.
Here's a link to some general information about mono:
http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0001617/
I was a school nurse for a time and saw a number of teens with mono. Some were on limited school for many weeks and some were at school, but came to lie down for a rest every day. Other teens were back to normal much faster.
I don't know any way to figure out which will recover quickly and which will take longer.
If his spleen is enlarged, there is also concern about rupture of the spleen, which can be very serious. So, he may not be able to ride anything except the very gentle rides. You will need to talk with his doctor to determine whether he can go or not and what kind of restrictions he might have.
Even sitting in a wheelchair may be more exertion than he will be able to do.
There is one company (Randys) that sometimes rents
ECVs to older teens on a case by case basis, especially if they have been driving a car for a while.
BUT, even though someone using an ECV is not walking, it is quite tiring to use one. The driver has to be alert all the time because people dart out in front of ECVs all the time. And, if he gets tired and can't drive, no one can drive it for him while he sits. So, I would stay with a wheelchair.
A wheelchair can be brought into all attractions to the point of boarding (except Swiss Family Robinson Treehouse and Tomorrowland Transit Authority, which guests need to be ambulatory to experience). As was already pointed out, a Guest Assistance Card will not provide a place to sit and it will not shorten the distance walked in most cases.
There is more information about renting wheelchairs in post 2 of the disABILITIES FAQs thread near the top if this board, or you can follow the link in my signature.
If he's tired and needs to lie down without leaving the park, there is a First Aid in each park with cots in individual rooms or cubicles. Some people in the past have reported being too tired to ride the bus back to their resort without taking a rest in First Aid before leaving.
If you do go, it may be a more 'laid back', lie by the pool type vacation, so I would go with that possibility in mind.