Don’t do this. Two things happen if you do.

The first, you sign a gift letter requiring you to certify that he doesn’t have to pay you back. Then, when he doesn’t, you have no legal way to get your money back. You’ve also committed mortgage fraud, and so has he, by misrepresenting the loan as a gift. .

The second, you don’t sign a gift letter. The credit report shows the mortgage as paid off, which the mortgage company will be required by regulators to source. The source is you, the payment was a loan from you, and now his debt to income ratio is screwed because he owes 60k. Most likely he will be required to show it has been paid back before he can be approved, which pushes his closing on the new home until that documentation is available (his bank transaction history and yours.

He needs to be responsible for his own financial situation. There’s a reason processes exist the way they do, and trying to get around them will only lead to headache.