The most “tactful way” to tell your son-in-law that you don’t trust him, don’t trust his marriage to your daughter, but want him to pay the monthly mortgage payments on a house that you refuse to allow be his (i.e. he should pay someone else’s monthly mortgage payments) and you want to enforce that he forever live in your neighborhood?
Just be honest with him. You truly give mother-in-laws the good name they are known for.