If you're talking about the kind that just sit over the penis (the plastic penis looking sleeve things) then yes they work, because they make his penis seem longer.
But given that there will be a thick plastic sheath over his penis, it doesn't feel better for him, he will lose most of his feeling. That's the only kind of penis extender I know of.
I would suggest trying different positions and techniques first, if you want your sexual gratification to increase.