If you are having to open his mouth and administer meds, I'd use this opportunity to slip in a PROERLY GUTLOADED cricket or two, even better a few fattier food items, such as silkworms, dusted with proper vitamins. Based on the photos of the chameleon 4 months ago, it doesn't appear to be dying of old age. Try getting him some outside time, as well. It's not going to solve his Vitamin A deficiency, but real sun can improve their mood and stimulate the appetite. Do your research, read the key information that was provided on gut loading and FOLLOW IT.
Get him on a proper supplement schedule that includes a product with preformed Vitamin A. I would not dust with vitamins until the stuff from the vet is finished, since too much is as bad as not enough. The information on whether to provide preformed A or rely on gutloading alone is sometimes conflicting, but unfortunately you missed both opportunities to provide vitamin A. In my experience, preformed vitamin A is needed, and I like to use Dendrocare for dart frogs, for montane species, since the dose is lower, safer, and it contains other crucial vitamins and minerals.