Did your family file the FAFSA form and request Federal student loans — in addition to the scholarships and grants? That’s really the first step in funding college, and your question doesn’t make it clear as to whether he has applied. Learn more at www.StudentAid.gov.
If he has already done that, he should go to the admissions office or the financial advisors at the college and explain that he cannot get more money for college, and ask if the have a work/study program where they could offer where he could offset some of the costs.
And I assume he has been to Scholarships.com to see if he can find more “free money.”
He will not get a different kind of loan (other than a Federal student loan) without you as a co-signer. And you should NOT do this, even for a ParentPlus loan, since you have two other children to worry about.
If the school will not make up the difference then he will learn a painful lesson about the need to plan and save. He will have to live at home and go to a community college, at least for a couple of years. (Making sure that the credits he takes will transfer.) You can be sure it will make him a better person in life.