i would have the vet check for tounge ulcer...if he has one,and you get him antibiotics quick...he'll survive,if you let it go...the ulcer can grow and nothing would be able to cure it.
that happened to my husbands sister's cat.
same thing...started hanging the tounge out...and sure enough it had a big ulcer under it's tounge.i would get it checked...or look yourself.
but i think if it's all the way out...i'd go to the vet ASAP.
Good luck and i hope it's nothing.