Chickens don’t like to be wet. Maybe a shallow pan of water for him to keep his feet cool would be nice, or you could install misters.
If you feel you absolutely have to give him a little dip in water make sure that;
-The water is room temperature that has been sitting outside in a bucket for a while & has acclimated to the heat. Water coming out of a hose or tap is colder than you think it is & the temperature shock could be life threatening.
-Don’t get his head wet.
-Don’t do it before he goes to bed.
-Consider not doing it at all. (See bellow)
Try to give him more natural ways to cool off like lots of shade, a cool dirt hole in the shade, cool snacks like watermelon or chilled, chopped up grapes, or frozen peas.
Make sure he has extra waterers & that you refresh them multiple times during the day. Keep your eye on the sun & move waterers & snacks to shady areas as the sun movies.
It gets extremely hot here during Southern California summers & misters are a life saver without getting the birds soaking wet or straining your water bill. Good luck!