If UVB is good, he's getting correct supplements and correct amount of food I doubt it's MBD.
I wouldnt worry too much if everything is good and hes walking fine, sometimes my leopard gecko will bask with both front legs behind her until you disturb her and then she gets up and walks away from you. (She has two heat sources, a heat pad 24/7 and then a CHE during the night)
This goes for my tegu as well, she will bask with her arms somewhat behind her occasionally.
My adult female bearded dragon likes to bask on her log sometimes with her arms dangling back a bit enjoying the warmth.
Maybe it's just a comfortable thing to do?
Although in that picture he looks like he's trying to scratch himself which is what my beardies tend to do when they are going into shed, have something on them, or are in shed.