1- Does the man(a converted Muslim) can convert again to his original religion?

-->In Islam NO!....but, still this is between him and his God.

2- Or if not can they have divorce and both can remarry?

-->Yes! they can, because there is divorce in Islam but that is the least thing that has been made halal by Allah for Muslim.

-->And if they are already divorced, for sure they can legally remarry.