If he can talk, ask him where he'd like you to start. ... I'd get the worst part over first since it gives you the heebie-jeebies and takes longer to dry. Then the legs, and upper body. Use a hair blower to speed up the drying (my pitbulls loved it), talcum powder, and a touch of cologne. He'll be grateful; and you'll be relieved.
Here are some instructions on wikihow. You need to look down a way to see where they are giving a sponge bath to someone in bed. They recommend to start from the top and work down, with the back and privates being the last things done.
wikihow/Give-a-Sponge-Bath