not a lawyer here, so you get what you pay for:
There's lots of ways to do what you want.
Contact the financing agency and apply to go through their process which they may, or may not agree to.
Have a lawyer write up an agreement between the two of you, which may or may not be any better, or legal, or enforceable than something you write yourself.
Write out an agreement between yourselves, FOLLOW IT TO THE T, and document every step, and you'll probably never have to take it to court and see how well you did.
Shake hands, make the payments yourself, and assume ownership in the end.
Give him the cash on his word that he will continue to make the payments.
Obviously these are listed in order from safest to least safe.