Not exactly sure what happened with your son's bull and I'm sorry to hear that he ended up losing it.
I don't know what you mean by not enough knock down power? If you mean energy transfer from bullet to animal, I don't think there is a much better bullet design than the cup/core boat tail, secant ogive, polymer tip style such as the ELDX. If you mentioned lack of penetration, I might be able to understand that.
Either way, I sincerely don't believe that it is possible to hit a bull elk in the vitals, including point of shoulder, with a 143 ELDX at PRC velocities and not have sufficient penetration, expansion, tissue damage, etc to very quickly kill the animal. My observation has been that a direct shot to the shoulder of an elk causes near instant disablement with death quickly following. I would also expect that you would lose a considerable amount of meat from both front shoulders due to bullet/bone fragmentation and resultant blood shot tissue damage.
My guess is that the shot placement on your son's elk was less than perfect, likely low, resulting in an elk with a single broken leg and no damage to vitals organs.------SS