Suppose he willed his car to his living son, his house to his son, and the remainder of his estate to Bob.

He dies. Son is alive. In this case, son gets house and car. Bob gets everything else.

Suppose son dies first. In this case, the house transfers to the son’s estate, where it is then transferred to son’s heirs. The house was bequeathed to the “son”.

But the car does not transfer to the son’s estate. The car was bequeathed to the “living son”. The car transfers to Bob with the rest of Dad’s estate, not to the son’s heirs.