You need to rank him up, the higher his rank is t he better your chances of getting legendary loot. Also legendary engrams dont guarantee legendary loot the whole idea is to increase the chances for legendary looks by adding the number of possible exotic items and legendary items to the pool with the rare and uncommon ones if I have a bag with 50 blue jelly beans and 75 green jelly beans then I add 25 purple jelly beans (this is roughly equal to a blue engram) your odds aren't high if you want to pull a purple one but if I add another 15 yellow jelly beans (rough equivalent of a purple engram) you still aren't guaranteed a purple but the odds of getting purple or yellow items has gone up so its not increasing the odds of legendary items per say as much as it actually increases the odds of you getting something you want