The FMV of the property is what it would have sold for on the date of his passing. A real estate agent should be able to assist with this, as they can look back in their records and see what homes that are comparible to yours, in the same general locale of your home actually sold for in 2015 around the time of his passing.
Take note that if you live in a community property state, then you get a full 100% step-up in the cost basis. Whereas otherwise you only get to step up his 50% that was left to you.
ANyone else reading this thread, if I'm backwards on the above, please let us know.