I'm sure it is not genetic, it is either MBD or neuro or combination of both, only vet can confirm this possibly after doing xrays.
i can tell you this, i very much respect the way you care about a frog, not many people would do what you're already doing for him, especially not younger people. I would be very proud mom if my kids would try to save a live being as much as you do.
i don't think previous owner cared at all, doctor or not and i don't think calling and asking if he knew will help at all, technically you're frog owners now, thus you're responsible for all expenses occurring.
I'm so sorry he is so sick and hope it can be fixed. please keep us posted. good luck!