@Tony Karns See what his number is. Try to get a best guess of what the current appraisal would be and the ARV .. without these, you may be throwing darts in the dark. Get a number (amortization) that works which will keep you with the ability to renovate successfully, while also providing a payment that will be assured passive income in his mailbox every month.
If he plans on retiring in 20. Offer a 20 year amortization, even a balloon if you know what would be a comfortable timeframe to pull your investment back out and pay him .. stay worst case in your own mind as far as time, since he's open.
The years can be adjusted if he is uncomfortable with any initial proposal. Key is to give him what HE wants and what you WANT. Win. Win.
When you make an offer, don't provide one for a yes or no answer, provide multiple scenarios as people in that scenario may choose instead of saying yes or no, gives a better opprtunity for a proposal to be accepted.