The water should be in the car anyway. What if a hose comes loose and he needs to refill the radiator? If you actually weighed the rest of the stuff, you'd find it isn't that heavy.
Nothing you have mentioned weighs anything significant for a car. Get the bathroom scales out and you can prove it.
But you know that already, as you say: 'at least in his mind'. You don't really believe a plastic bucket weighs anything for a car. Is it more the bulk he is complaining about? The way to win him over is to involve him and use his suggestions.

You too need walking shoes; THAT YOU HAVE WORN IN. And weatherproof clothes. I simply wear Dr Martens, which are good office shoes and street walking shoes. I could only pity people at work when we had the July 2005 bombings in London and people found themselves limping 5 miles home in their eigth of an inch thick soles. The sniggers at 'Mr Be Prepared' suddenly stopped.
This is something not mentioned much on ETS, but I'd swap any gear for a waterproof in the winter. Your knives and multi tools will do you no good, when you've died of exposure. I've found Chris Townsend, who writes 'the backpackers handbook' gives good advice. He recommends Paramo gear which he says are the most breathable and also the only breathables you can smash through the thorns in and will still stay waterproof.

'No blade of grass' and 'the long road home are fiction'. Would you really be mugged for your transport? Why not put up a post asking if anyone knows of this happening? You and I are virtually the only ones using this thread and I don't know the answer.
The Sock