OP's previous comment from 3 weeks ago:
How does it work the other way when a dad has his son every weekend. He works full time Mon to Friday and spends every weekend with his son willingly. The mum wants to go away for the week but doesn't want to take the son, and the dad has no annual leave to take and tells her this, she drops him at his dad's on Friday and when the dad drops him back Sunday evening she's not there and has gone away on holiday, where does the dad stand ?
Your credibility is pretty poor to be honest OP. You knew about this holiday, REFUSED to have him, refused to take annual leave (why not take parental leave instead?) and she went ahead and did it anyway, it's not quite the same as you being blindsided by it, if this is from 3 weeks ago, what did you do and where is your son now?
It's definitely not right, but I think you should have posted from an honest point of view to begin with, as it begs the question, why are you not prepared to take any annual leave/parental leave during the school holidays to spend with your son anyway (regardless of what his mum is doing/planning)? How can you say you can offer him stability if you won't even take any time off during the holidays to spend time with him? And you've already said that you struggle to take him to school in the mornings as it interferes with your work.
Fobbing him off on your family for childcare is not stability and is not a solution, by all means go to court, but make sure you are doing it for the right reasons, being angry at her for wanting to go on holiday and for going out with her friends is nothing to do with you and nothing to do with what is best for him, it just sounds like you are trying to exert some control of what she does by pretending you have no flexibility, when you obviously do.. surely any time she would ask you to see more of him should be a blessing not an inconvenience, and any normal parent would jump at the chance??
If your job is so inflexible that you genuinely can't even handle getting him to school on a Monday, with notice the evening before (the reason is irrelevant), then you don't seem the best person to have him during school time.