Boys.
Gotta love em'.
How about, while he is out there getting dirty. AFTERWARD... you use the hose or sprinklers to wash him off???? Right there in the yard.
Its easy that way. And fun!
When my boy was anti-baths and anti-washing hair... we would, go outside, and play, And I would actually give him a 'bath' outside, with the hose and sprinklers. I had soap and wash cloth and towel all right there handy. SO much easier and he got clean and didn't even know I gave him a bath!
We have a wall around our house, so no neighbors can see in.
But it was effective.
Then he went onto other phases.
I just used the hose/sprinkler method for a bit. Occasionally.
It was not a habit forever.
I let my boy, explore in the yard. He has so much fun!
And when he goes outside, I put on his 'home clothes' that are already stained (and I can't get out the stains), so I don't mind if these clothes get dirty anyway. Because I don't put him in good clothes, when he goes outside.