Need based aid will be reduced if he has less need (ie. receiving outside scholarships). Merit aid most likely won’t be reduced as long as his total (merit + scholarships) is less than COA but that depends on the specifics and small print of the merit offer. There are some ways that a student can increase their COA. For example, I think once in 4 years they are allowed to spend aid on a computer.
Many outside scholarships are for one year. If he receives several, he may be able to defer some so that he doesn’t receive them all his freshman year and go over COA.
Finally, some outside scholarships will send the check to your son personally and not the school. He is supposed to report those funds to the school but some don’t.