Education vs Business Investment

I have a son and daughter both in their twenties. My daughter went to university which my wife and I were happy to pay for, viewing our children's education as part of our obligation as parents. Our son wasn't academic but instead decided to set up his own delivery business. He has done well but needs some investment for expansion. He has asked my wife and I if we would put into his business the same amount we paid over our daughter's three-year university course for fees and living expenses. I am not sure if investing in a business is quite the same as going to university. What is the fair thing to do?