I’d also do some research to your son’s school and where you can use each type of meal. (There is probably a phone number or email for the food service provider on the school website). Sometimes there are hidden pros/cons.
As an example, at my D’s school the block meals allow you to eat at some of the other (non-cafeteria) places on campus. They have some meals set up to count as one “block meal” (ex. a sandwich, chips, and drink at the sandwich shop would be one block meal). In contrast, the 21 meal/week plan limits students at her college to eating at the all you can eat buffet style cafeteria.
Another thing to find out in advance is if your S can change is plan once he arrives at school. Many schools give a two week or so grace period to change plans. I put my D on the 21 meal/week plan initially she asked me to change it her first week on campus and it was no problem to do so.
Lastly some schools have arrangements with off campus eateries where they take flex dollars. If that is the case, it is a big plus for getting some flex dollars (the kids feel like they eat out while still using meal plan money). I’d ask about that at your S’s school as well.