It sounds like you're doing everything correctly. You can try feeding it and getting it back up to a healthy weight, but that frog has NOT been eating regularly. They were either lying to you or it has something seriously medically wrong with it. My guess is they were lying. Also, in the pic you shot from above it's holding his arm oddly. Almost holding it bent backwards. May just be the way he was sitting, or could be an early sign of MBD.
I don't want to sound pessimistic but you may want to make use of the return policy...

This really ticks me off. If a store was keeping a dog or cat in that condition and sold it as healthy, they'd be up for animal cruelty. Heck, you could report them anyway since you have photo documentation of the condition you bought the frog in. Might not do any good, but at least it'd force them to go through some paper work.