Oh no, I'm so sorry to hear about that. I really hoped he'd get better.
Do you want to euthanize him so he isn't suffering or do you think it won't be much longer anyhow? :( If you choose euthanization, there are basically three ways to do it.
1) The fastest and most humane and THE most difficult to carry out is decapitation and destruction of the brain.
2) Clove oil. There are instructions elsewhere on the forum about how to use an overdose of clove oil but I'm not sure exactly where they are. You could probably google it, too.
3) Take him to your veterinarian and have him or her do it. My personal favorite.
If you have to euthanize at home or he passes at home, be sure not to flush him down the toilet. The best way is burial, cremation or disposal in the trash bin. I know, all of this sounds so heartless and cold. :( But it's necessary info, regrettably.
I'm sorry that we couldn't save him.