Wow.....interesting situation!
I'm no expert, but I guess I'd give it a day to see if he can work it out himself. If it's still there tomorrow, I'd consult a vet if possible. If you CAN'T get a vet's suggestion, then use a huge net and pull the fish out of the water, then remove it with tweezers while holding him on his side. He should be fine for about 2 minutes without water. Get some help if possible, and don't worry about man-handling it a bit...that's a HUGE fish, and they are quite rugged. Just make sure you place him on a wet (damp) surface, or in a large tub of some type (bathtub is fine) with an inch or two of water. Make sure your hands are wet if you are handling him too. Plan to get wet!
I'd rather be a bit rough and do it quickly than be overly cautious with handling it and end up dropping him when he shakes back and forth (and I'm sure he will). Grab him firmly and do things quickly and decisively. Get it over with as quickly as possible and get him back in the tank! If it does take more than 2 minutes. Try adding more water (all water needs to be de-chlorinated) from a 5 gallon bucket, or get him back in the tank if you absolutely have to!

He will probably go through periods of powerful spasms (shaking) and laying still, just time it and do the best you can.
Like I say, consult a vet for a more educated opinion, if you can!
Best wishes!