There's been too many obvious hints since the 1st episode that Tate is dead. You see Tate standing behind the baby monster and it flashes between normal Tate and him when he was wearing the skull face paint that he had on when he killed those kids at school. He appears covered in blood in Ben's office too briefly.
Also as mentioned before, Tate should be in his 30's by now if he was still alive.
Constance mentioned that all her kids seemed to suffer from some sort of autism bar one that was of physical perfection as she put it. Possible Tate has assburgers or some sort of high functioning form of autism. Her wording suggests to me that she had more than just Addy & Tate, so I wonder what happened to the others if there were more?