Firstly, a 38-gallon flexarium is way too small for a fully grown adult, which is almost what your cham is. That size is suitable only for babies really. By now you should be moving him into a 175 gallon one.
And although it's a small cage, if all you've got in there is a rubber plant, it might be a bit sparsely planted.
See here for some examples of suitable set-ups:
http://www.chameleonsdish.com/habitat/setup.htm
What UVB light are you using? (2.0, 5.0, 10.0... what brand?)
Your temps sound fine.
Your gutloading is not adequate. Bran and fish flakes are not suitable gutloads. Gutloading is not about keeping the insect alive - it's about passing essential nutrients on to your cham. You need to be gutloading using a variety of fresh fruit and vegetables (carrots, squash, oranges, etc.) as well as nutritious leafy greens (like kale, collard greens, dandelion greens). In addition to that, you could use a well formulated gutload mix, such as those available commercially, or a home-made one like the
James / Wells / Lopéz Gutload
For more about gutloading, read
Gutloading Basics
Nutrobal contains vitamin D3 (as well as other vitamins) along with the calcium. Chameleons can overdose on Vitamin D3. You should be using a calcium supplement that does not contain any D3 at most feedings, and only use the one with vit D3 one or twice a month (depending on how much natural unfiltered sunlight he gets exposed to - less D3 the more sunlight he gets).
These issues may be contributing to a general lethargy in your cham which may be responsible for his reduced appetite.
What are his activity levels like nowadays? Are his eyes open and alert throughout the day?
When last did you take him for a vet check up or have a fecal test for parasites done? (parasites could cause watery/mucousy stools - but that could also just be from the fact that he is not eating and only drinking).