It depends on how you plan to do the deal. Do you plan on going to a bank and get a mortgage? In that case, they'll have an appraisal done, and your sales contract would have to mirror something close to market.
If on the other hand, you give him a note, or take it on "Subject to" if there's a small mortgage, then I think you'll be OK.
Another way is for him to deed you half the property, add you to the deed, you now own half. then wait a while, you buy his other half. When he does this, he may choose to report gifting you the half. Many people doing it does not report the gift.
Maybe I’ll have him deed me half the property. It seems like a lot less of a headache.