M.G.
Been there... gross lol. We had one of those DVD players on the back of our car seats and our toddler projectile vomited and all I cared about was that DVD player (brand new). Poor kid. He didn't know what hit him and I was scrambling to get that thing off the back of my head rest ...
We had these liners that were flannel like that had holes to fit the straps through in our infant seats - but I see you can order similar things in a more heavy weight kind of material and even mesh from amazon.com if you type in carseat liner.
Here's one -
https://www.amazon.com/Bebenuvo-Premium-Cushion-Stroller-...
I'm not sure it would offer total coverage but at least he'd be able to sit on something and be safe.
Here's another that says "Super absorbent"
https://www.amazon.com/Meeno-Absorbent-Protector-Universa...