I know from a close friend whose son came back his sophomore year too early that once it is there it is there. He got it during basketball and came back for playoffs a week early. He got sick and missed the first several weeks of baseball. He got some version of mono every year until after he graduated. I know of several others that it took several years to completely beat because your immune system is weakened. If he is playing college, he needs to take 6 weeks to heal rather than just 4. No reason to risk it.