Hi, I'm sorry he's got MBD, but it's mostly due to the type of UVB tube you have, and how you have it mounted and setup.
It's most likely PetSmart's house-brand, called "All Living Things", which are extremely weak and are also made in a factory in China that churn out thousands of tubes and sell them to many different brands, all the same tubes, all very weak, and all emit harmful light rays as a byproduct of the cheap manufacturing. The main issue is that #1) They are extremely weak UVb tubes to begin with, #2) It's a T8 strength UVB tube, which are the weakest type and cannot sit on top of a mesh lid, and #3) It's Too far away from his basking spot for a T8 strength UVB tube.
There are 2 different strength of UVB tubes, the much weaker T8 and the much stronger T5. For any desert reptile the T5 is obviously recommended, and it has it's advantages not only for your dragon, but for you and your wallet.
The T8 tubes (15 watts-17 watts typically) cannot be obstructed by anything, they must be within at least 6" max distance from his main Basking spot/platform on the Hot Side of the tank, and they must absolutely be replaced once every 6 months, as their UVB decay rate is very fast.
In contrast, the much stronger T5 UVB tubes (24 watts-26 watts typically) can sit on top of a mesh lid (Though with a dragon already exhibiting symptoms of severe MBD I suggest mounting it under the mesh lid anyway), they only have to be within at least 11" max from his main Basking Spot/Platform on the Hot Side of the tank, and they need replaced only once every 10-12 months, so you save a small fortune in this regard.
The 2 brands that are highly recommended for Bearded Dragons and used by most experienced owners and breeders (because they are very strong, emit no harmful rays, and the UVB spectrum they emit is perfect for dragons) are the Reptisun 10.0 brand, and the Arcadia brand. (DO NOT BUY THE REPTISUN 5.0 BRAND, THEY ARE ONLY 5% UVB TUBES/LIGHTS AND MUST BE MUCH CLOSER TO YOUR DRAGON, TOO CLOSE FOR THEIR EYES...THESE ARE MAINLY FOR TROPICAL REPTILES). Both of these brands are best ordered online rather than buying them in a pet shop, as I know that Petco and Petsmart only carry the much weaker Reptisun 10.0 T8 UVB tubes, and just an 18" T8 tube costs $37.99 in both stores!!! It's ridiculous, especially since you can almost purchase a longer, much stronger 22" Reptisun 10.0 T5HO UVB tube AND and 24" T5-rated fixture for the same price on Amazon.com.
I highly recommend that you buy a T5 strength UVB tube for your dragon and get it mounted underneath the mesh lid. I'd totally skip the weaker T8 tubes, as you'll not only spend much more in the long run, but because he cannot even move, meaning his MBD is very advanced, he needs the T5 UVB tube. You'll also have to buy a new tube fixture that is rated for a T5 UVB tube (24 watts versus the T8-rated tube you already have that is rated for 15 or 17 watts max only), but again, on Amazon.com, which is the cheapest place I've found and where I have ordered my tubes and fixtures for years, you can buy both for under $50 shipped.
If you want to spend a bit more money you can also buy a 24" Arcadia 12% T5 UVB tube, it's a 26 watt tube and a bit stronger
UVB light than the Reptisun 10.0 T5HO tubes, but either will due. The Arcadia tubes are made in Europe and cost a bit more, that choice is yours.
I recommend just going onto Amazon.com ASAP and ordering a 22" or a 34" (skip the 16", too short) Reptisun 10.0 T5HO UVB tube, and then either a 24" or a 36" T5-rated tube fixture for it. BE SURE THAT THE FIXTURE YOU BUY INCLUDES A METAL REFLECTOR THAT SITS INSIDE IT BEHIND THE TUBE, THIS IS VERY IMPORTANT BECAUSE IT'S THE ONLY WAY TO GET
UVB LIGHT REFLECTED THROUGHOUT HIS TANK.
If you go to the "General Discussion" forum here I posted a topic called "Amazon.com links for Reptisun 10.0 T5HO and fixture" just a few days ago, it includes a link for the 22" Reptisun 10.0 T5HO UVB tube and a 24" T5 rated fixture that has a metal reflector with it. I have 3 of this brand/model of T5-tube fixture and they work great, no issues at all.
In the meantime, while you're waiting for the new UVB tube/fixture to come, I would poke holes in the mesh lid and then use either long, plastic zip ties, shoelace, wire, twine, etc. to strap your current UVB tube fixture to the underside of the mesh lid, it's pretty easy to do. Make sure that you do it over the Hot Side of the tank, and leave enough space so that your bright white Basking Light can sit right alongside the UVb tube (THEY NEED TO BE RIGHT BESIDE EACH OTHER AND BOTH OVER THE HOT SIDE OF THE TANK, NOT END-TO-END, BUT RATHER RIGHT ALONGSIDE EACH OTHER). They move whatever you use as his basking spot/platform right directly underneath both lights, so that he gets both of them at the same time while basking. THIS REPLICATES NATURAL SUNLIGHT AS CLOSELY AS POSSIBLE....ALSO, BE SURE THAT BOTH LIGHTS ARE ON EVERY SINGLE DAY FOR AT LEAST 13-14 HOURS MINIMUM!
When the new UVb tube and fixture come just cut the zip ties or whatever you used to strap the tube fixture on the underside of the mesh lid, remove the old tube, and put the new UVB tube into the new fixture and then strap it to the underside of the mesh lid and turn her on...
Be sure to dust her live insects at least 5 days a week in a proper calcium supplement for reptiles (I'd be dusting every day of the week in calcium once the new UVB tube comes, at least for 3 months or so, then scale back to 5 days a week in calcium). I'd be dusting in a Reptile Multivitamin at least 3 days a week as well, 4 is probably better for the next few months as well, then scale back to 3.