Sorry for the long post!
First, I would like to say AWESOME JOB on how quick you were, but also with great quality. That cage is fit so well you could use that for one of them in the future.
Second, I can tell you first hand everything people are saying here works and is correct, if it's not it is quickly corrected, usually by multiple people. Don't listen to what pet stores tell you. Trust this forum/vet/someone experienced with chameleons from now on haha. When I was about 11-12ish I had a female Jackson's chameleon. Of course, at 11 or 12 I did not do the proper research for the animal (easily avoidable with this forum, it's fantastic!), and she unfortunately passed away.

She was also gravid and had 14 little babies and because of the same issue, they passed too. I gave it a second go around now that I have income (once everything is initially setup and purchased, it's pretty inexpensive to raise one), responsibility lol, and this forum I have now had a perfectly healthy Jackson's chameleon for about 6 weeks now. But I had pretty much the exact same setup and routine you had initially (how I remember I don't know lol) and Jackie had the exact same symptoms and appearance.
This time around I did the research, and got a vast majority from this forum.
-You've already got a
large screen cage - awesome!
-Heat lamp is okay, what wattage is it at?
-The compact UVB really needs to be a linear UVB 5.0 by ReptiSun, or ReptiGlo if that's all they have for now. I know it's more money, but this is essential. You can look at craigslist for some stuff. Id get a new bulb though, unless the owner has the receipt and can show its less than 6 months old.
-Mist consistently! You could try to get a rubbermaid type container to catch the water underneath for a drainage system. I noticed my chameleon will only drink after I have misted for about 2 mins consistently. The shower recommendation I've read all over the place, just bounce the water off the walls.
-The Gatorade/Electrolyte thing is new, but I haven't read about what to do with a sick chameleon. the members who recommended it though I have received advice from and have seen multiple times.
-It does appear as if he may have MBD. Use the search feature here and you can learn more about it. But basically he needs natural UVB that the sun gives off, emulated by the linear UVB bulb (crucial), otherwise bones and other parts don't develop properly.
-It's great you are giving him calcium on your crickets. someone may have posted here, but search on the forums for the correct calcium with vit. D3 and without. Jackson's are more sensitive to dusting.
DONT GIVE UP! If he is still moving, there is hope. Just keep consistent with the recommendations. Hope everything works out and keep us as updated as you can
And please fill this out, it will help a lot!
Chameleon Info:
Your Chameleon - The species, sex, and age of your chameleon. How long has it been in your care?
Handling - How often do you handle your chameleon?
Feeding - What are you feeding your cham? What amount? What is the schedule? How are you gut-loading your feeders?
Supplements - What brand and type of calcium and vitamin products are you dusting your feeders with and what is the schedule?
Watering - What kind of watering technique do you use? How often and how long to you mist? Do you see your chameleon drinking?
Fecal Description - Briefly note colors and consistency from recent droppings. Has this chameleon ever been tested for parasites?
History - Any previous information about your cham that might be useful to others when trying to help you.
Cage Info:
Cage Type - Describe your cage (Glass, Screen, Combo?) What are the dimensions?
Lighting - What brand, model, and types of lighting are you using? What is your daily lighting schedule?
Temperature - What temp range have you created (cage floor to basking spot)? Lowest overnight temp? How do you measure these temps?
Humidity - What are your humidity levels? How are you creating and maintaining these levels? What do you use to measure humidity?
Plants - Are you using live plants? If so, what kind?
Placement - Where is your cage located? Is it near any fans, air vents, or high traffic areas? At what height is the top of the cage relative to your room floor?
Location - Where are you geographically located?
Current Problem - The current problem you are concerned about