If you can do it, put him in a bag and chop his neck with a heavy knife. I know that doesn't sound real good, but it's instantaneous, and he will feel nothing. Sorry.
If it needs to be done the quickest humane death is removal of the head. A sharp pair of scrissors should do it on chicks Gamebird shears or fish boning shears will work better. Hold firmly over a sink, cut just behind the head severing the spine and blood vessel, point down and hold tight. They will spasm a bit and a little blood will come out and it will be over. Far more humane than any idea of freezing (horrible way to put down any animal including fish) or gassing. If you can't look at the head you can lay a paper towel over them and use your fingers to guide the scissors/shears to just behind the head and cut then hold it down to the bottom of the sink until they stop moving. That's how I had to do my first few since I couldn't stand seeing their heads come off.
Others have mentioned though some things that might help the chick. So long as it's struggling and making any attempts to swallow liquid on it's beak or food it comes in contact with I would keep trying to straighten it's legs and keep it's energy up using the suggestions you've been given. You'll know when they've reached their limit and stopped trying to progress.
Little guy died yesterday in my hand while was searching for info! It did take it out of my hands but I need to find a way or get used to doing it to save any other chickies from suffering. Unexpectedly DH as offered to do the deed in the future, but he may not be around at the right time so I still have to brace myself to do it.
I do plan on processing some of out excess birds when they are older but that is such a different mindset, you know the bird has had a good life, has had a purpose and there isn't the cute chick factor. Plus I'll probably wienie out and have an amish neighbor process them.
I delivered 850 chicks to a neighbor on Sat - in a two door Focus (earsplitting din I tell ya!) and thought - gosh, I can't even raise one out of this batch! Ya have to give credit to the hatcheries. I don't think most folks would be eating chickens and eggs at such an affordable price if it wasn't for the commercial hatcheries.
Thanks for all the suggestions, I'll be working my courage up in the future!
Mine was bought used, is now almost 10 yrs old (I've had it 9), manual trans, no power anything, NO A/C!, has over 130,000 on it, tempermental radio, gets the oil changed when it makes funny sounds, nothing more major than brakes and some radiator work on it. I am not too fond of it as is sure isn't very comfy (seat is getting alittle "broken in") but I think at the rate it is going it'll probably be running for another 10 years with my luck! And summers in humid MD suck!