My barn manager/ owner has a grey/blue roan coloured gelding and she used a blue shampoo
Maybe use this magic brushes? They will help get rid of dirt that's deep down and loose hairs
Try brightening shampoo, I used it on a great it makes them supper white. Dover sells it, the shampoo bottle is purple
If you don't want to bathe him you'll probably just have to clip him often so that there's no grass stains or any other stains etc.
Lucky Braids Whitener works really well
I swear by the corona shampoo.