The only child he can claim as a qualifying child dependent for all the tax benefits from that type of dependent is the child that you are both the parents of. He can file as Head of Household if he paid more than half the costs of keeping up the home you live in together and claims his child.
You must claim the 3 children that are yours as qualifying child dependents. Your fiance can't claim any of them because they are your qualifying children. Does their father have any rights to claim the children based on any legal agreements?
