I am not qualified to offer medical advice so don't think what I post is in any way intended to do that, but most of the above posts above must be somewhat disheartening for you. There are definitely degrees of severity with mono and hopefully your son recovers more quickly than some of the boys referenced above. My HS Freshman son was diagnosed with the flu, strep throat, and mono about 2 weeks after his team started practice this year. They did check the spleen which was not enlarged. He was told to sit out for a week, which was probably as much for the flu as the mono. His only instructions were to shut it down if he felt fatigued. We made the coach aware of the diagnosis and he left my son out of the more strenuous activities for an additional week after he returned, but my son was able to participate fully in all other areas of practice with no fatigue. 2 weeks after his diagnosis he went back to full participation and had no ill effects for the remainder of the season.
It obviously sounds like your son's case is worse than my son's, but try to stay optimistic. Hopefully the effects won't be as bad or as long-lasting as some of the cases discussed in this thread. Good luck to your son.