If you look at his legs (as gesang said) you will see he is showing severe signs of MBD...and he can't even hold his body up. He needs to go to a vet NOW!
The vet can give him injections of calcium and when the blood calcium levels are high enough, a shot of calcitonin to draw the calcium back into his bones.
In the meantime, you need to address his husbandry.
What brand and type (long linear tube or compact/spiral) UVB light does he have on the cage?
Is the T-rex the only supplement? Once a week is not often enough, IMHO.
I use a phosphorous-free calcium powder at almost every feeding. Most feeder insects have a poor ratio of calcium to phos. so this helps to make up for it.
I also use a vitamin powder that has a beta carotene source of vitamin A twice a month. Beta carotene won't build up in the system. There is controversy as to whether the chameleons can convert the beta carotene so some people give a little preformed vitamin A once in a while. Preformed vitamin A can build up in the system and the excess can prevent the D3 from doing its job leading to MBD.
I also dust twice a month with a phos.-free calcium D3 powder because my chameleons rarely get sunlight. D3 from supplements also can build up in the system, so don't overdo it.
Appropriate basking temperatures will aid in digestion and thus play a part in nutrient absorption.
Its also important to gutload and feed the insects a nutritious diet. I give the crickets a wide assortment of greens (dandelion, kale, collards, endive, escarole, mustard greens, etc.) and veggies (carrot, squash, zucchini, sweet potato, sweet red pepper, celery leaves, etc.).
Phosphorous, calcium, D3 and vitamin A all play important parts in bone health (as well as other important parts in the body) and need to be in balance. When balancing them, look at what you feed the chameleon, what you feed the insects and what supplements you use.
Some of the damage incurred by your chameleon already will not be reversible...but if things are brought back into balance he could still live a long life.