Try to hit the coropted glowing things around his body. Ice you have destroyed all, it should fall down on the ground knocked out. You do not need any taming food for it, Just go up to it and click tame to tame it. No need to build a saddle because it already comes with one.
Pro tip: Don't use explosive weaponry and really bad rexes with no saddles like the devs did. Fab Snipers for taming it with little/no collateral damage, actually decent rexes + imprint + saddles to kill it