I'm going to tell my mum the truth about why she can't take my son on holiday without me and my wife
Is there a reason why your mom can't take your son on a holiday without you and your wife being present?
No way! Do you even want to know?
While some families take separate vacations, most others do not and prefer to take them as a whole family. If you don't want your son going on a separate vacation with just your mom, that is okay to refuse to do so unless you and your wife are present. Having a joint family vacation can be important for a child and it's okay if you want to have vacation memories of going as one large family.
Who cares what most people do, I want my mum to make a big deal out of it.
Why would you want to have a huge problem with your mom? It would be easier to just tell them that you want to go as an entire family, as that sounds reasonable.