I haven't heard of LED fixtures using any UVB. It would definitely have to include other lighting with the LED in order to provide it. I use LED's, not because they're a fad, but because they stay cooler than florescent lighting and I get amazing plant growth. I do not want anything throwing off heat on the dart tanks. LED lighting is also a great option for my red eyes, as I have albinos that cannot have UVB. Not to mention, the lifespan of the lighting is excellent. The one on my grey tree frog tank has been there for several years now. However, for my tortoise, he has both a mercury vapor light for basking and a fluorescent tube uvb light to provide a broader area of uvb. If you're using a coil UVB light, they have to go to a very concentrated area in order to benefit from it. All uvb lighting should be replaced every 6 months to maintain its effectiveness and should not be placed over glass.
It sounds like MBD to me also but you cannot rule out potential infection either. MBD can be treated so that it does not get worse, and you may see some improvement in symptoms, but any deformities cannot be reversed. Honestly, I would probably return him. If you do not want to though, I would keep him quarantined and if you do not see any improvement over a few days, I'd start looking at other sources of the illness. Do you have a video of the frog's behavior? That might be more telling.