Its complicated but I think:
1. It is a household of two and the joint expenses need to be allocated between the taxpayer and Fiance;
2. The child support paid to the mom is debatable – the IRS will try to refuse it because it is not court ordered but he is legally responsible to support his children. I would consider looking at the state child support laws and see if there is a minimum by law and use that as the basis for allowing these payments.