Our kids have trouble expressing what they're feeling so they would do behavior things. Is it possible your son doesn't like the long ride for maybe some reason- boredom, feeling car sick, they don't engage with him during the ride? These could be possibilities. You could try having the grandparents engage with him maybe do a sing along, talk to him about things, can the grandparent who is not driving maybe watch a video with him or do something with him ? Do you think taking a break in the middle of the ride might help? Maybe they could stop for a snack ? This also occurred to me what if your son has to go to the bathroom during the ride? Would he let them know?

oh and tell grandad to not react

kids just do shit autistic or not. its all about testing their boundaries. if they have some they feel secure. If my autistic granddaughter spat at me I would spit right back at her. Then say how did that go for you? My granddaughter is non verbal so her means of expressing herself can become quite extreme, but thats ok. You just need to understand she is way more frustrated than you are, way more overwhelmed and coping the best way she can.

Ash did this, I went autism store n bought a whistle and a toy that u blow in and it makes bird noises, this stopped the spitting, I had deal toy sounds but I liked that over the spitting

Thanks for your answers. It is behavioural more than sensory as if I was to do the journey with him it would not happen. He has distractions with him - his iPad and some teddies, and there is music in the car that he will pick and sing along to. Those are what I use with him and there are no issues.

The issue has to do with grandad explosive reactions when it first happened.
My son tried spitting in my car once but because I ignored it he stopped and never did it since. I just wondered if anyone had a similar issue and used rewards to stop the behaviour and what type of rewards or strategies worked for them.
Grandparents are starting a visual reward chart, which hopefully will work. The issue sometimes is it draws more attention to what he shouldn't do and this can increase his anxiety and exacerbate the unwanted behaviour.
Spitting in a cup or tissue also doesn't work as his anxiety is towards people's reactions, so the behaviour he displays are usually "targeted", based on how the person reacted in the past.