All righty, so for the most part you're on the right track! I see three main issues that are important to correct at your earliest ability:
(1) He is well beyond the age and size that he needs to be moved into his adult-sized enclosure. You'll want to upgrade to 24"x24"x48" minimum as soon as you can.
(2) You are unfortunately using the incorrect type of UVB bulb for your guy (this is one of the commonest mistakes for people newer to the hobby and it's an easy fix). The compact bulb you are using does not distribute UVB light in a way that chameleons benefit from it and if you continue using it, he will develop metabolic bone disease. What you want to use instead is a linear T5HO UVB bulb which is as long as the enclosure is wide. I would recommend upgrading the cage size and getting the correct lighting with that, so you don't waste money on an 18" UVB bulb when you will ultimately need a 24" one
(3) And lastly, your gutload for your feeders. Cabbage is not recommended to use at all and neither is spinach. Spinach binds calcium which you definitely do not want to happen, as calcium is critical to your cham's health. Kale should also only be used sparingly as is, like spinach, is a cruciferous vegetable and will bind calcium. Collard greens are an excellent choice, though! To that, I recommend including things like mustard greens, dandelion greens, escarole, sweet potato, and papaya. Check the resources section of the forum for other good options.
Hope that helps!