Q: Am I within my rights to withhold a month's child support if he'll be with me that entire month?

A: If your child support order does not already include a waiver of child support while your child is with you exclusively for a month or whatever period of time you have him, then no, you cannot unilaterally withhold child support or take it to defray your costs. Only another court order modifying the existing court order can legally change your obligation, so you remain exposed to your ex filing for contempt or to enforce the child support order. However, if you and your ex sign an agreement allowing you to do this, and you draft the language to cover this type of arrangement going forward, you can file a consent order modifying the existing child support order that makes this change (e.g., "the parties agree that the father's child support obligation shall be suspended for the month or prorated month(s) of summer visitation exercised by the father each year"). Alternatively, with her written agreement in hand for this one occasion, and assuming that you trust her not to file a motion accusing you of violating the order, you can take the chance that such an agreement will hold and you can get away with it. The court will do nothing without her filing a motion about it. With such an agreement in hand, you would have a reasonable argument to defend your ex's later filing against you of she decided to do that, but technically you cannot by agreement legally change existing child support orders without the court's approval in a new order.