he gets overrun once you go too far up that alley(the one where he says "i'm being overrun" so, don't go up that alley until you have found him and he will survive

once you get to the house with him in it, destroy the apc outside the door go meet him and he will run off let him get ahead of you then once he is out of sight, then you can go to the cathedral and he should stay safe
as for the flame tank, destroy it before you get anywhere near rescuing him