Child support question?

My fiancé and I live with my in laws. We have an 11 month old daughter and baby #2 on the way. My fiancé had a child from a previous relationship. He gets dropped off here at 6 am and doesn’t get picked up until 8-9 pm Monday-Friday. He spends the whole weekends here. My fiancé and I signed him up for school here nearby the house because the mom didn’t want to sign him up for school anywhere. My fiancé picks him up/drops him off from school on his days off or if he’s working I pick him up or drop him off sometimes my mil does when I have appointments and can’t make it. We buy him food, school uniforms, shoes, school supplies, we take him with us everywhere. To the park, out to eat, to the mall stuff like that. My in laws take him out too. We take him to all the family parties with us. Point is when the mom found out I was pregnant again she decided to put child support. They want to charge $450 a month. But he literally just goes over there to sleep on the week days. Is there anything we could do? I just don’t think it’s fair that he has to pay $450 a month when he’s not even with her. She doesn’t do anything or provide anything for him. We went to the child support office and they were just so rude and didn’t help at all with anything. It’s stressing me out and my fiancé because we literally have no idea what to do. Any help or suggestions would be great!