CG for part of year

I have a prospect with the following scenario -

Two companies, one a partnership 67/33 owned, and one an S corp, 75/25 owned respectively until May 1, then 100% owned by the majority owner.  As of May 1, 2020, the partnership effectively dissolves in an asset sale.  The S corp has lots of profits for this year and probably a couple of years going forward.

Any thoughts on how to set up a DB for the (one man) S corp?  Can we "just" make it effective May 1?

I think a primary concern is the "effective" dissolution of the partnership; it's probably in existence thru Dec 31.  But no discrimination problems since the only other person potentially in is an owner.

Put in a de minimis accrual for the other owner, use 3-year cliff vesting and disregard vesting service prior to the effective date.