I'd try setting him up on a plant either outdoors or in the shower and gently misting him (ideally mist/small droplets directed upward that can fall onto him) for a period of about 10-15 min or so. Sometimes it take a few min to get over the initial shock and discomfort of the water, even if they are thirsty. If he is thirsty, he should start to drink and I'd mist until he stops, then try again in 5 min, repeat. Ten to 15 min is enough time to determine if he's thirsty and if he remains relatively uninterested, he's fine.
FWIW, my Jackson's here on windward Oahu don't drink everyday and drink less than I recall as typical for my Veileds when I lived in MI. My feeling is that this is largely related to humidity--RH rarely drops to 50% here, and only on the hottest days does it dip lower, and is typically >80% at night. Having kept chameleons here in a semi-humid, semi-rainy climate and in MI where humidity is generally low indoors for half the year due to heating, I am acutely more aware of the importance of appropriate humidity for these animals. When humidity is typical of their natural environment (for most chameleons that means quite high) they only need to drink small amounts here and there. Desicating environments (e.g., screen cage indoors with home heating or air conditioning) require them to drink large amounts of water, frequently. I think the hobby has, at least in the US, focused too little on providing appropriate humidity (while avoiding mildew!) and too much on providing drinking water, which becomes a requirement to make up for the low humidity.
cj
