I dont know if the tail will grow back but if it does it will most likely be wavey and irregular. That's what happened to one of my boys after he got a bad case of finrot, he lost about half his tail and it has fully regrown.
Some people reckon the curling is caused by hard water and can be remedied by a product called 'Trace' which I can't seem to find a link for, but basically it replaces all the trace elements in healthy amounts like zinc, copper etc. To be honest it sounds somewhat questionable but its worth a shot if its important to you that he has the best regrowth possible. I am sure there are many versions of this product on the market.
Good luck with it all, I can't get over how lucky he is to have gotten you!!
Does it impede his swimming at all?