There seem to be a lot of panthers that have issues that start off with eyes closing. I'm still not sure why although a some people say that it could be to do with a lack of prEformed vitamin A. Excess D3 can also create quite a few problems...and the A and D can interfere with each other if not in balance too. Caution needs to be taken with dealing with prEformed vitamin A and D3 from supplements since they can both build up in the chameleon's system. (See more on that below.)
Is the minerall the indoors (D3) one or the outdoors (no D3) one??
Male panther chameleons seem to need quite a bit of food and hydration. If you look at the pads on the top of its head and they are full/protruding rather than flat or sunken in it seems to give an indication that both the food and hydration levels are good....just my opinion from keeping them for many years....nothing scientific to back it up that I've found so far. You indicated lethargy for about 3 months, which IMHO means there is something going on.
Here's some information that might help concerning supplements and the reasoning behind it...
Appropriate cage temperatures aid in digestion and thus play a part indirectly in nutrient absorption.
Exposure to UVB from either direct sunlight or a proper UVB light allows the chameleon to produce D3 so that it can use the calcium in its system to make/keep the bones strong and be used in other systems in the chameleon as well. The UVB should not pass through glass or plastic no matter whether its from the sun or the UVB light. The most often recommended UVB light is the long linear fluorescent Repti-sun 5.0 tube light. Some of the compacts, spirals and tube lights have caused health issues, but so far there have been no bad reports against this one.
A wide variety of insects that have been well fed and gutloaded should be fed to it. At that size you only need to feed it every two or three days. Feed it enough that it doesn't get fat (and, of course, doesn't get thin either).
Since many of the feeder insects we use in captivity have a poor ratio of calcium to phosphorus in them, its important to dust the insects just before you feed them to the chameleon at most feedings with a phos.-free calcium powder to help make up for it. (I use Rep-cal phosphorus-free calcium). Not sure how to tell you to dust WC insects if that is what you will be doing.
If you also dust twice a month with a phos.-free calcium/D3 powder it will ensure that your chameleon gets some D3 without overdoing it. It leaves the chameleon to produce the rest of what it needs through its exposure to the UVB light. D3 from supplements can build up in the system but D3 produced from exposure to UVB shouldn't as long as the chameleon can move in and out of it. (I use Rep-cal phos.-free calcium/D3).
Dusting twice a month as well with a vitamin powder that contains a beta carotene (prOformed) source of vitamin A will ensure that the chameleon gets some vitamins without the danger of overdosing the vitamin A. PrEformed sources of vitamin A can build up in the system and may prevent the D3 from doing its job and push the chameleon towards MBD. However, there is controversy as to whether all/any chameleons can convert the beta carotene and so some people give some prEformed vitamin A once in a while. (I use herptivite which has beta carotene.)
Gutloading/feeding the insects well helps to provide what the chameleon needs. I gutload crickets, roaches, locusts, superworms, etc. with an assortment of greens (dandelions, kale, collards, endive, escarole, mustard greens, etc.) and veggies (carrots, squash, sweet potato, sweet red pepper, zucchini, etc.)
Calcium, phos., D3 and vitamin A are important players in bone health and other systems in the chameleon (muscles, etc.) and they need to be in balance. When trying to balance them, you need to look at the supplements, what you feed the insects and what you feed the chameleon.
Here are some good sites for you to read too...
If you can't access the sites above that have the word "archive" in you can do it through the WayBackMachine.