There is still a gift involved. You won't have to pay gift tax, but you will still need to file Form 709. Your gift will be the amount that you purchase for him, the inheritance will be your portion he receives at the time of death. The basis of the gifted portion will be the adjusted basis, assuming it is the lesser amount, which it usually is.
Under joint tenancy, both partners jointly own the whole property, while with tenants-in-common each own a specified share. Buying a property as tenants in common also allows them to leave their share of the property to beneficiaries other than their partner when they die.
Tenancy-in-common interests receive a step-up in cost basis for tax purposes. For example, if two tenants in common own a parcel of real estate and one dies, the decedent's interest in the property will receive a step-up in basis but the surviving tenant's interest will not. So the survivor receives the basis of adjustment for the inherited portion, but not on his own.
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