If you don't want to bring him to the vet to have him put down and you're concerned about him feeling pain or something, dogs respond to a variety of "downer" drugs in a similar way as humans (google it to be sure) and you can get him to relax like that then do the deed.
http://www.pet-informed-veterinary-a...uthanasia.html
I doubt you can get pentobarbitone sodium but other bartibtuates can usually be found and used to relax the animal while the deed is done.