Howdy Nicole,
Take a look at the UVB tube and let us know the mfgr and the style. You may see something like ZooMed 2.0 or 5.0 or 10.0 or some other brand and type. This could be an important data point.
If you are using a typical trigger-style sprayer that requires a trigger squeeze every time you want to squirt water then I suggest picking-up one of these mist bottles for about $7 at Home Depot:
It's the middle one in this photo...
http://rlflomaster.com/sprayers/hand.htm
You can partially fill it with warm water (gets cooler when misted), pump it up and mist the enclosure plants and him until he drinks. It may take 5-20 minutes to trigger his drinking process. He will get used to the warm mist and enjoy it after a while. It is very likely that he needs a lot more access to mist water than he has been getting. This may help quite a bit. Think about an excess water drainage plan for your enclosure...
I' a little concerned with the supplements. The ZooMed vitamin has a lot of things in it. Vitamin A is one of those vitamins that can be overdosed in chameleons. Some need it much more than others but I think that Jackson's are one of the chameleons that are sensitive to O.D. I'd suggest that you switch to Miner-All (I) as a source for Calcium with vitamin D3 and use Herptivite multivitamin as a source for vitamins. Also pick-up a can of Miner-All (O) as a source of Ca/with no D3 to use once in a while instead of the Ca/D3 product. Try Ca/D3 once a week and Vitamins once every two weeks. Maybe add Ca/no D3 once a week.
The photos will be very telling...