Hi.

I'm playing hardcoe mode, and i managed to save somewhere where my companion (Boone) is poisoned.
How do i cure him? All he do now is dying on me. I simply cant give him an antivenom?

I think they fixed that problem in a patch, prior to the patch, companions who were poisoned died instantly if you gave them stimpacks and antivenom could not be aplplied to them.
So, do you have the latest patch? If not, then all you can do is wait out the venom and hope he doesn't die or reload.
Tip for RPG's: Save often.

Stimpacks until the poison wears off.