Ok, if I understand correctly, you are the spouse of his biological father, who is the non-custodial parent?
Whitman’s cost of attendance is about $63k, so it looks like his mother and stepfather are contributing very little, if anything. Whitman is listed as using CSS Profile including non-custodial parent finances, so you presumably had to fill that out for him, right?
You may be able to reverse-engineer an approximation of their finances with Whitman’s net price calculator at https://npc.collegeboard.org/student/app/whitman , although it may be less reliable if merit scholarships are mixed in to the actual award.
It may be best to start by first doing your financial planning so that you know your price limit. Of course, there are the complexities of negotiating with his mother and stepfather in terms of shares of college money to pay, but you need to know where you stand beforehand, and he needs to know what the price limit is before he makes his application list (so that he knows that admission with too high a net price = rejection).
It may also be best to nudge the initial college search toward schools with significant merit scholarships, due to the uncertainty around need-based financial aid based on divorced parents.