The more plants, the better for him and for increasing humidity. Please be sure to wash the leaves with dish soap and water to remove pesticide/fertilizer residues.
Safe plant list links are at the bottom of this page:
https://www.chameleonforums.com/care/enclosures/
I can't say if it is the problem but is very easy for these little guys to become dehydrated.
Lethargy and eye irritation can be symptoms of dehydration and too low humidity.
See if he is drinking enough and add enough plants to get the humidity between 70-85%.
Jackson's can be a little slow to drink. I find that creating a fine mist in the air with a hand spray bottle helps to encourage them to drink but try to avoid spraying him directly.
In nature they drink droplets of water off of plant leaves.
He should be drinking more than once a day.
Another possibility is that the UV light is too strong for him.
I have been told by several experienced breeders to use a UVB bulb that has already been used for about 6 months so that its UVB output is weaker.
Increasing the distance from the light to him will have the same effect as a weaker bulb or even adding an extra layer of screening beneath the light will significantly reduce the amount of UVB that he receives.