Billy...,
I used Hornady's 154 SP Interlock in a 7mm Rem Mag for years and many elk. They always produced dead elk for me. The Sierra that you have been using will also do the job. As will many other bullets. If you will be more comfortable with a tougher bullet the Interlock has a good reputation. If you would like something tougher then look to a bonded like Accubond, Swift Scirocco or Aframe to mention a couple.
You will find a lot of posts that swear by or at specific bullets but there are none that will make bad shot placement into a good shot and few that will not do the job at hand if the shot placement is reasonable.
I wish you and your son the best of luck with this year's season and your planed Elk hunt.
8mmwapiti
P.S. Elk hunting can be addictive, and there is no known cure.
