"sometimes before I go to work and some on the weekends" does not sound like 50% of the time to me. Parenting is a 24-hour job, especially with a child still in diapers. You're expected to buy supplies such as clothing and diapers aside from any child support you pay.

Please remember that child support is for the child, your child. Working full time may not make financial sense for his mother--what she'd pay for childcare would probably eat up all of her paycheck and then some. $85/week is a relatively small amount.

You certainly can file a motion to vacate the order, but be prepared to be denied. In that case you'll simply be required to keep paying. Spending a few hours a week with your child does not relieve you of the responsibility of helping support him. Think about what kind of father you want to be.

Most states have child support calculators online. Google your state's name and child support calculator. Enter your income, the mother's income, the number of children involved, if there are any child care or medical insurance costs and anything else it asks. This will let you know about what the court is going to go with. Good luck and best wishes.