Son will get a live chickenpox booster vaccine soon. i've had pox twice, apparently my immunity to it is low. will i catch it from his live vaccine?

Unlikely to matter: If the kid did develop a mild rash & illness the agent would be the weakened vaccine virus and not wild chickenpox. If you were exposed to that strain it would be a free booster to whatever immunity you have. Many who think they had it twice were mislabeled in the past & had a variation of hand foot mouth syndrome, which looks similar.