What happens if a King with a younger brother gets his wife pregnant then dies before his son is born ?

This has happened several times in history. Someone- either the uncle or the mother is declared "regent" for the baby until he grows up. You can look up "regents" and find examples of times when this happened. The child is still the presumptive king. 

It's not going to happen troll.

A person is appointed as temporary  'regent'  to govern a state pro tempore because the regnant monarch is a minor, is absent, abdicated the throne, is incapacitated or dead

the Unborn Son Becomes King

Up to parliament.  But probably the brother would be Prince Regent until the child was old enough to take the throne.

The son becomes King.