For Christ's sake,take him to the vet instead of experimenting on ways to kill him. The pet was a faithful companion and THINKS you love him more than you love a few dollars.
I had a cat I had rescued some years before come crawling back home after being gone for 3 days. 3" hole in her side with ribs sticking out of it and crawling with worms. I HAD to empty a .22 pistol into her brain,and it WASN'T fun at all.I had to instantly put her out of her misery. But,it wasn't very instant looking. I didn't want her to suffer another hour getting her to the vet. There just wasn't time in her case,but there IS in yours. My pets all get taken to the vet when the time comes,and are given a decent burial at home,NOT thrown in a landfill.
I am not rich,but in the last month I had to spend $500.00 each on 2 cats. One got hit by a car and hip dislocated(lucky it wasn't more serious,he'll be o.k.) The other is a 14 year old female I got from the SPCA. I thought she was acting senile,but turned out was some sort of infection. She's better now.
Some people should not be allowed to have pets OR children. Take care of them if you're going to get them in the first place. It is your humane duty. Let the expert handle this.