Clove oil is a humane way to euthanise him. You just mix some with warm water, add it to a container with water from his tank (use a container you can bin afterwards, you don't want clove oil near your tanks or equipment). Then move him into that container with the clove oil in it. Swimming in the clove oil mixture will gradually sedate him, and then you add in more clove oil/warm water to overdose him and put him down. It's the same sort of method that a vet would use to put down a mammal, sedation then overdose, so they don't suffer. I have used this method, and it does work and seems peaceful.
You can get clove oil from most chemists, pharmacies, or some health stores, since it's often used by people for toothache.
Alternatively, I've also folded a piece of paper towel, placed the guppy on that and folded the paper towel over them then hit the head hard with a heavy object. Not fun or easy to do, but it is instantaneous, which to me seems kinder than a drawn out, painful death gasping. I prefer clove oil, but I used the other method when I didn't have any, it was late at night and the fish was clearly suffering and beyond saving.
I know nothing about a baking soda method either, but I'd be wary of the idea. Baking soda is very alkaline, and I assume it would hurt the fish.