Our son was charged with domestic and has never been in trouble before . How long will he serve?

This depends on the facts in the case - how badly was the other person hurt, was there property damage or was there a restraining order violated? If there were truly no other prior incidents in his criminal history he will unlikely serve any long term sentence. Probably a suspended sentence or probation that would hang over his head. If violated in the future would cause his incarceration for some time.