I’ve learned from experience that it’s important to do things right — and if you don’t, you are tempting fate! There should be a promissory note in place, even if your dad isn’t charging any interest. Here’s a link to NOLO –where you can download the forms.
Since your father could “gift” the money there is no tax implication for either the lender or the borrower. But it would be a good idea to document the transaction with a paper trail in case the IRS questions it.