Congrats on the offer. It's nice to get a decent offer from a school that was high on the list.
As Fenway mentioned, you are going to want to spend some time talking to financial aid at the school.
Right now it is hard for them to give you an exact number because he still has a couple of years to go - however they can tell you what type of financial awards certain GPA/SAT numbers can get you typically.
That can give you a good basis for things your son needs to shoot for in order to lower the costs as much as possible. My oldest is an academic kid, so by the time junior year rolled around he had a good idea of what he needed on his SAT and GPA to get the max money from his prospective colleges.
Also, make sure what the offer actually works out to money-wise. Because it is offered as a percentage, you need to find out what exactly the percentage applies to. Is it just tuition, or does it include room and board, books? You want to make sure you are comparing the same things if he gets other offers.