I think the underhandedness,and omitting you in their decisions is definately disrespectful and not right as far as sensible goes.
I would definately NOT allow my child to go. I wouldn't trust with all the info given here, and there is so much you haven't told us.
You must be strong, for your child to have faith,trust and respect in you as mum, no one should try to take your place. You must be strong and stadfast for YOU. You are MOM.You say what goes, how and when with your child.
Is there a reprt to say your ex has fully recovered to be able to make even such as a 'request' for your son to go on holiday without you - WHY are they not inviting you?
Don't let anyone else decide what's for your child.
Stick with the legal advice, as often as you can, with as many concerns that you have.
Take as many precautions as you can that they cannot do such things behind your back; keep sons passport,report a fears and worries to passport agencies/border police,AND DEFINATELY DO NOT let them take him go on holiday without you.It's out of the question and totally unreasonable with the circs.
I think you have very good cause for concern and you should stick to your mum instincts. Don't let them try to walk over you.
You can tell them straight and politely,"I understand you want to take my son on holiday,but I think it's unreasonable in the circumstances especially as I am not invited (or can afford to go)..." TAKE A NEUTRAL friend who is willing to act/listen as witness in court. And when you get chance, write what happened down.IE who,where,when,how,what.How it made you feel and how you feel they responded.
STAY STRONG.SAY NO. your child will be obviously upset with you for a short term, but better keep him with you that something undesirable happens. Your child will understand, and you must try to explain well and truthfully.Tell him you are concerned for his safety and it's only right that you stay with mum at this time,there's plenty of time for holidays when he's older, just not now.