It’s also worth noting that Northeastern does not consider subject tests and AP scores are ranked towards the less important side. Agreed with what @TexasMom2017 and TomSr said as well too, it’s really a bit of a crapshoot in college admissions, and a rejection in no way means the student wasn’t qualified, just that there was not space and other factors were not what they were looking for. And again, NU.in is not a rejection, and can actually be a really great option for some/many if you can afford the extra cost first semester 
Another thing to consider: What did your son mark to the NU.in preference question on the application?