twoinanddone has a point. You are likely to find that your son does not come home as often as you think. Coming home often can be very disruptive to a student schedule. Both socially and academically. Frequent visits by the family can have the same effect. Image if you went home to your parents or they visited you frequently. Same idea.
Every kid is different. You don’t know how this will play out.
If money is tight, ask yourself what happens if he doesn’t come home as often as you think. Will he have enough food. Granted it is nice to save money by using his plan for meals when you visit but it might be better to play it safe for the first semester and then pick the best plan for second semester. At that point, he will have a better idea of what the food options are available and at what cost.
I can tell you that eating all his meals in the dining hall gets old very quickly. I doubt he will eat all 21. He could have class conflicts or just decide not to get up for breakfast.