There's no reason he can't play ball, do well in school, and join the legions of us who went pro in something other than sports.
There are 168 hours in a week.
The average semester load is 15 class hours.
Good students average 2 hours homework per hour of class time.
Young adults need 8 hours of sleep per night.
168 - 15 - 30 - 56 = 67
If he wants to play baseball, he could explain to his mother that choosing to spend those 67 hours in the company of goal-oriented young men under the supervision of coaches who care about his personal development and success while he does something he loves, builds lifetime friendships, develops his potential, and gets great conditioning will be wiser than the decisions that 95% of his classmates make about how to spend their time.
If he wants to twist the knife a little, he can say, "You're right, Mom. If I don't play baseball, I'll have a lot more time for parties and indiscriminate dating!"
Meanwhile, you can take the point of view that ending one's baseball career before the game requires him to do so is a decision that should be left up to the player.