Can we fight getting full custody if the mother is half paralyzed?

My husband received a child support order yesterday and I don't understand why because we have offered to buy him the things that he needs or take him to the store to let him pick out his things but she is not happy with us buying him things. She wants the cash instead and I honestly don't see why it would make a difference if we just bought him those things. She has been rude. She has been deceitful to myself and my husband. She constantly puts him down. When we go see him and her house and a very unhealthy home. It's dirty. It smells you can't go to the bathroom without at least 10 Nats flying around you. It's very cluttered and she always buys junk food. He is 9 years old and he weighs 140 pounds. This is because she has nothing but junk food and lets him eat whatever he wants. The doctor even said he was overweight and then he has to run for an hour every day. We have no problem buying him what he needs and taking it to him. We also would like full custody because my husband basically raised him from birth because the mother is half paralyzed.