The normal recommendation for a shower is to put a plant in the shower, turn the shower on and point it at the wall so it bounces off the wall onto the plant. Get the water so it feels hot but not burning coming out of the showerhead (it will cool down as it travels). Turn the shower off, put the chameleon on the plant, turn the water back on. He's likely to act like he hates it at first, but should relax in a few minutes. Leave him in as long as you can. 20, 25 minutes would not be too long.
It's possible he ate something from the plant's "dirt".