The survival instincts of the tarrasque he's basically merged with understand things like 'slashing your throat' or 'taking poison' but are not sentient enough to figure out that he's trying to raise people up who can kill him.
Here's the thing - in an RPG, a secret is no fun if it never gets out. Secrets exist to be revealed. So, at some point, the players have to learn what the whole point of this exercise is.
So, somehow you're eventually going to have to communicate that this guy has, in effect, a second personality that does not allow direct physical action, but somehow will ignore conscious planning. And you'll have to make it plausible to them.
I, personally, have a problem with the idea that this thing can recognize poison, but not recognize the setup. But that's me, and your players are not me. If you think it'll fly with them, go for it.
For that matter, he's tough enough that slitting his own throat is not something he can just casually do anyway.
Dude, even Chuck Norris isn't so tough that he couldn't kill himself.
