All you have to do is enter the stepped up basis as a completely separate asset. For example, if current basis being used for depreciation is say, $200,000 with $50,000 allocated to the land, that means your depreciation basis is $150,000.
If the new basis is say, $400,000 after his passing, then keeping percentages the same that would be $300,000 for the structure and $100,000 for the land. The difference is what you would enter as a completely new asset with an "in service" date the date of his passing. So that would be a new asset with a $200,000 cost basis and $100,000 allocated to the land.
In some states you only get to step up the share of the deceased. That means you'd enter your your new asset using half of the values above.
