Easy tame; just make a pitfall trap with 2 wall high walls and 4 foundations. Get it to follow you through some ramps to the pit and get it stuck. Just make sure the Rex is actually stuck (they can sometimes glitch through the walls). Once you see a little drop of the Rex, he should be for sure stuck and you can tame safely. Make the pit out of stone though so you have yourself a durable taming pen that you can rely on to get multiple of these guys.