“If he’s denied everywhere in the end besides UMASS …”
If he is denied everywhere in the end except for an acceptance to U.Mass Amherst, then he will still have a very good university to attend in September. I have worked with many graduates from U.Mass. I know a fairly lengthy list of graduates from there who are excellent coworkers and who have done very well in life.
You only get to attend one university at a time. If your safety is very good (eg U.Mass), then you do not need to apply to a long list of safeties. If your safety is very good, then 1 acceptance and 23 rejections still leaves you attending a very good school, although the 23 rejections are going to hurt.
I do think that university admissions in the US has gotten insanely unpredictable. Higher ranked schools will deny you because you to not add whatever they want to their student body even if you are above their 50th percentile for test scores and grades and have good references. Lower ranked schools will deny you because they are afraid that you are using them as a safety and they want to protect their “yield”. I feel lucky that we live close enough to the border that we can have a safety in our in-state public university and more safeties to the north or east of us in Canada.