@OneMoreKid - Your child will be in good company wherever they go. There are always top students who couldn’t afford top schools or (surprisingly) didn’t get in to them. IME, those students find each other at the school they end up in. Keep looking forward as to the adventure ahead and I hope it’s a good one!
I think that you need to tell him that there are outstanding kids that will be smarter than him anywhere he goes. The name of the college doesn’t correlate with how intelligent some of the students can be. Many students have personal situations, and even if they were admitted to tip-top colleges, they couldn’t go for many obvious reasons. These selective students are enrolled in many types of institutions, and what to speak of the outstanding college that he has been admitted to.
If the plan doesn’t go well, then he should go to his backup school, which is perhaps an amazing option. As others have noted, college-admissions is arbitrary from the on-looker, because not only are there priorities of an institution, but perfect applicants just like him are flooding everywhere, and they just can’t take them all. It’s the luck of the draw, so a stellar student’s rejection from a particular school means although that he/she was qualified by general objective and subjective measures, there were other factors that were simply not in the applicant’s control.