He is rumored to have be in contact with 2 teams lately: Williams and Mecedes.
Following the rumors that links him to Mercedes, the voice has been turned off so it seems to Mercedes for him.
Redbull offered 1 year extention to Perez, he asked for 2. So at the moment Redbull has no second driver for the next year.
It is likely that Sainz will join Mercedes next season - to fill the slot left by Hamilton. Sainz is not a bad racer, compared to Charles, he has a slightly superior performance, but it's just that at some points his luck runs out. I'm sure that his joining Mercedes will allow Mercedes to maintain their performance and it is possible that they can get a podium, but only if it is accompanied by good upgrades by Mercedes on their car. I'm quite positive about the possibility of Sainz moving to Mercedes, and I hope it happens.