Pancuronium bromide would work.
It's what they use during lethal injections. There are many doctors who believe that it does not knock a person out, but simply makes them unable to react. The same thing would happen with Vecuronium bromide, or any other neuromuscular non-depolarizing agent. I'm assuming the guy wants to induce a conscious coma, and then hurt them, rather than simply killing them with the drug, so things like tetrodotoxin is out (as would be most other poisons which occur in nature, which not only induce a conscious coma, but also kill the dude).