Lawyers will provide a free 15 minute consult. It might be worth having one look at the wording in the IEP. It's not FAPE if he has to have a parent on the trip and that accommodation is going to cost you $1500. FAPE should be free to families. If he needed a paraprofessional to go on the trip, the school would pick up their tab. You're saving them money because you are providing your time at no cost.
I'd ask them what costs will be covered. If you don't ask, I'm positive they won't offer to help with the added cost of him needing a 1:1 chaperone. PA Consult Line is another place to ask what financial help the school should provide so his accommodation can happen. https://odr-pa.org/consultline-contact/
I'd put it in writing if you ask for financial help: XX has on his IEP that a parent must attend field trips so he has access to them given his disability. This year, the trip is to NYC and the added cost for a parent to attend with him is around $1500. Since schools are required to provide FAPE to students with IEPs, how does XX School District cover this? Should I pay my way and then send receipts so I can be reimbursed or will XXSD pay directly for XX's accommodation of having a parent attend? What costs will the school cover? I'm assuming I will need to pay for my own meals but the school will cover transportation, lodging and admission fees. Please let me know so I can figure out my finances. Thanks.
My wording is inspired by Lisa's recent article on not being soft with parent correspondence with the school.