Walk at a distance around the field. Get him to sit every so often, like you did with the bench and vehicals, and say, first few mins he stays sat and doesn't try anything - give a treat. then walk a bit more.

Each day or so get a slight bit closer and to be sat that bit longer.

Do becareful to not let him in the field, you don't want your dog being kicked and you don't really want to be the problem of the horses becoming scared of dogs.

That happened to my mums horse - the owner wasn't paying attention - their little dog ran out and started nipping his legs etc and then was petrified from then on.
But I have known of other horses just booting the dog...