Should my son pay me back for college?

My son went to college where he had scholarships. He lost his scholarships three times. The first time, we had just had a huge family loss so we stepped in and paid the difference. He agreed to pay us back, that was part of the deal. The last time, he was in senior year. Also, we told him not to take out unsubsidized loans. We felt that we had done it ourselves and it left us massively in debt. Back when my husband and I were in college, the interest rates were much higher so by the time we graduated, the loans from freshman year had doubled. We told our son he could pay us back instead, interest free, rather than racking up a huge debt to pay some bank or the government.

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He has been out of college for almost two years now. He has been earning good money now. His student loans, which were a very small amount, will be paid off before Christmas. He is talking about buying a $40,000 car. IF he had not borrowed money from us, I would not be bothered by this. But hearing him talk about buying a $40,000 car makes me feel like he can and should pay us back.

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But I feel like a horrible schmuck asking him to pay us back. What should I do?

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edited to add: we have younger children who still need to go through college so this money took from what we can pay for his younger siblings. As in, I expected to pay SOME for college, but the above and beyond amount is how much he should pay back.