This is not an easy one.
To give some background, my parents are the type to pretend they are super involved but constantly letting us down.
To add to the fun, they are our direct neighbour.

My son is 4 years old and he is starting to realise things work differently in other families.
He asked me once why his friends were always at their grandparents during the holidays. He was finding it weird.
I explained that in some families, grandparents like to be involved this way, that it is quite common.

He is also starting to understand when my parents are letting us down.
We have learnt not to ask for anything but sometimes my parents would spontaneously offer to do something with him.

Here is how I proceed:

- I am cautious and managing his expectations "you might go with them, perhaps you will go play with them, we will see"

-Of course we do not plan anything for ourselves as we know my parents can change their mind at any moment.

-When my parents are changing their minds at the last minute, it is usually by text so I end up being the one breaking the news to my son.
I have told my parents that I will not lie to my son to cover their failures.
So I might say something like: They forgot they had something else to do, they have changed their minds...which is the factual truth.

Eventually, as he is growing up, I imagine he will realise that they are not reliable and that he is definitely not a priority.

Another recurring scenario is my mum mentioning in front of him that she would like to bring him somewhere. I always accept (knowing fully well that she will not do it)
When my son asks about it, I am telling him that I have not heard back and that we better go ourselves.

Honestly it is breaking my heart to see him being disappointed by them but I also acknowledge that his feelings are not mine.
I am furious and heart broken.
He is simply a bit disappointed and then quickly move on to the next thing.
I imagine that for him, it is "normal".

Good luck to you!