Personal loans (and commonly, gifts) can be acceptable as long as the source of the loaned funds can be authenticated (that is, where did you get the money to loan in the first place). Your son will also need to prove he can pay off this loan.
You can either make a loan or gift the funds to your son. If a loan the son will have to show he has the financial means to pay back the loan. You will also have to authenticate the lawful source of your funds which was loaned or gifted to your son.