It looks like an injury to the base of the spine/beginning of the pelvic girdle, i.e. broken bones that have not set correctly. I don't envy that frog but it's likely he'll be fine like that. In any case, it won't improve. It's possible he metamorphosed that way but without an X-ray it's impossible to know (and it won't help). I don't think it's a deficiency but do make sure you supplement his food correctly.