This was lost in the shuffle, but
stone to flesh isn't a spell in 5e.
Flesh to stone is, however; but it does not exist on the poor sorcerer spell list.
Flesh to stone is a warlock or wizard spell.
Concentration is appropriate to the spell. The first save creates the restrained condition and subsequent saves against becoming petrified do not change that until the 3 save requirement is met. Not being able to break restrained for pretty much the rest of the encounter is decent enough that it should not be spam-capable or stacked with effects in the battlefield, hence concentration.