The court order sounds clear to me - if camp is not free, he pays 40%. Take eiram’s suggestion and see if unexpected visitation requirement will wash with the camp. If not, find another camp for your son to go to once he returns and request 40% reimbursement from your ex for that.

If there’s nothing in the parenting plan requiring a specific amount of notice for his summer parenting time, you’ll just have to roll with it.