It will most likely not grow back if it is completely gone, and so long as it is healed over (i.e. no scabs or anything) it will not affect his abilities to eat, climb ext... in the future. I doubt a groomer accidentally clipped an entire nail off, more than likely, another bird bit it off at some point or he had an accident. IF it is not healed over (i.e. you see a scab, blood, discharge ext where the nail should be) it is probably best to see a vet to prevent infection. The inner toes on parrots are shorter than the outer toes, thats just how their feet are. Unless there is an open wound, the most likely reason he keeps that foot up is because it's his less dominant one. Most parrots rest with their dominant foot on the perch and the other tucked up.
P.S. 2 of my parents birds are missing talons. The cockatoo bit off the females amazons talon, and she repaid him a few years later (they do not interact anymore). Neither have suffered any ill effects from missing talons, and neither of them ever grew them back in the 20+ years since they lost them.
