"God bless you" is what you say when someone sneezes.
You could say "God bless him (or her or them)" when talking about a third party who is not present. In that usage you might hear a "may" before the expression.
Honestly, other than for sneezing, you won't hear "God Bless you" used much, at least in the United States, unless by religious people talking to other religious people or religious people proselytizing others. Another time you might here "God bless you" is when a homeless person asks for money or in response after you give a needy person something.