This is MBD(Metabolic Bone Disease). The frog is not recieving a well balanced diet. Calcium+D3 will halt the progression of the disease, but the jaw may never fully close if the frog has stopped growing.

Drooping lower jaw is the first sign. The excessive droop appears that this frog has been suffering from MBD for a while. If not MBD then this frog has previously suffered from a jaw injury.

You need to start feeding this frog every other day and dust the feeders with Calcium+D3 every other feeding. Use multivitamins no more than once per week and not on the same day as a calcium dusting.