So have you gone to court already to establish your custody and has he been ordered to pay child support already? Or are you just in agreement about this before going to court?
I think it is wonderful that the two of you can do this peacefully; however, child support is supposed to be for the welfare of the child, so why would you want to waive it? Circumstances may change down the road and even though you may feel that money isn't needed now, it could be later on. You could always put that money aside for a college fund, etc.
If you haven't gone to court yet, then I am not sure if a judge will let you waive it. Part of it may depend on how much parenting time your ex spends with your child, i.e. how many, if any, nights out of the week/month your child spends at his house. Even if you don't want the money, you ex has a responsibility to financially provide for the child too. If the judge does order child support and you still really don't want or need it, then just send your ex back any child support money that he pays each month. That way he's not violating any court order, and the court does not need to know you have returned the money to him.