In all honesty I have never heard from Columbia that students leave debt-free. Ever! In fact, I cannot imagine ANY college ever making this claim.
FWIW, I will say we have found Columbia Financial Aid to be extremely responsive and supportive. Therefore, I think you should actually be contacting Columbia FA about this. Instead of just "reasonably" expecting what you expected, get a letter together and impress upon Columbia what your stiutation is. However, even with two kids in college, if you do make a certain level of income Columbia may, unfortunately, "reasonably" believe you are in a position to pay both tuitions. If this is NOT true, WORK with Columbia on this.
And, if your son is planning on grad school, he should be working with his advisors and the career office to look into graduate school funding and fellowships ASAP! Many students receive outside funding for grad school -- depending on field...-- in the forms of grants and fellowships.