OP - you won't get much sympathy on here, as has already been highlighted.
From the threads I have read, the general consensus from the Mum side is CM is for the basics, and you should share the extras like trips etc.
Like you, I have my kids OOW Fri - Mon, plus 1 night in the week plus holidays, in total nearly 50% of the time.
They are in secondary school so are quite independent and make their own way to / from school.
My exDW works P/T plus gets U/C plus a 4 figure sum from me each month, in total she nets over £3.5k into her account.
In addtion to that, I pay 100% of her mortgage, have bought her a car and paid a decent six fgures into her pension, so she has very little in the way of costs.
She has more disposable monthly income than I do, however she still wants me to pay half for school trips etc
I normally pay up as the kids put a guilt trip on me via what their Mum has said about me being mean, but it is bloody frustrating.
She's taking them on 2 holidays this year, I can't afford to take them on one.
It's not fair in my opinion, but my advice is to suck it up and that way nobody, including your son can ever hold it against you.
When he's 18, you won't be under this financial pressure anymore - I get it, it's hard, just don't let your exDW emotionally blackmail you via him - be the bigger person.