The first offer is never the best. We went back and forth several times with Admisssions at several different schools. In the next few weeks and month, money can come open. As students who they have offered, choose to attend another school, some of the money tagged to that student becomes available. I believe in some FA circles it is called Melt. Not going to post the URL. but look up Financial Aide and Summer Melt. There will be some articles.
How did your son's academics match up to the rest of the attendees? If his stats are below a typical student then it is not a surprise. I would also check with the schools and see if he can get invoted to Academics or honor days. These are usualy a long weekend when they bring students to campus to compete for scholarship money. Be aware he has to usually have very good grades or be in the top 10% to get an invite. This is another way to get additional money. It also allowes my son to remove a school from his list. He was invited to one of these scholar days. Went and did very well. He was in the top 3 or 4 in math and science. Yet he did not get any additional aide. He was competative in the other categories. Ticked him off really.
Getting the financial offer is just the beggining. Now it is time to start haggling like you are buying a used car. You will know quickly how badly the school wants your son.