Mine don't like to be outside alone either, and they like to wallow in the kiddie pools no matter the season. I leave them on our covered porch and leave the house screen door open, so they can see me while they dry. At first they were confused, but they eventually got it.
Do you have a door you can leave open and a way to keep him near it, maybe a tether if you don't have a porch to close off? You could spend some time training rather as if you were crate training-- leave him for a short time, maybe with a treat, return quickly when he's quiet. Repeat, adding time as needed.