Hi!
If you can get him into warm water (about 90F is what I used for my ducklings until they were about six weeks old), just deep enough to touch his belly, watch him closely, get him out before he gets too tired (mine would just stand around staring off into space for a few seconds, which I interpreted as pooped after five to ten minutes), and bring him into a warm, clean ,dry area to dry off,
he can go swimming any time. Mine started in a cake pan with an inch and a half of water when they were two days old.
A few of my runners didn't start preening themselves right after a bath, so I picked them up and dried them with a dry washcloth.