I also think you should probably tell him.

But you know your son better than all of us. However, Is option #2 even a viable option? How connected was he to this snake? Kids can be pretty observant, he might notice that the "replacement" snake is not the same in pattern, disposition, etc....then you might start a chain of little white lies and I think that will make you fill more guilty about your mistake in the end. Even one little innocent question from him may be enough for you to feel some guilt.

As other have said you could try to turn it into a learning experience.....but that's easier said than done with something this emotional.

Sooner or later though you will want your son to have some reponsibilty with the pets and you can use your mistake as an example of what can happen if your not careful.

Good luck and remember not to beat yourself up no matter what you decide.