I have a local butcher which also sells Raw mixes. They sell chicken, beef, turkey, and duck in 5lb rolls that are made up of 70% meat, and 30% bone. at $75 for 50lbs that's an amazing price in my book.
Wish I had a deal like that around me!
If the mixes your butcher sells are just muscle meat and ground bone, then you can just add in the various organs to balance out the meal. You definitely need liver and I strongly recommend green tripe. Other than that, kidney, spleen, stomach, testicles, eyes, etc are all good things to add.
The liver is the only thing I tend to spend more on. I get my liver organic because the liver is the organ that filters out many toxins so I prefer it to be as free of commercial toxins as possible. Though that of course is completely personal preference.
I find chicken and pig feet, pig snouts, and even beef tendon regularly at my local asian market and they are also good things to feed occasionally as they contain a good amount of glucosamine for healthy joints.
Much of raw feeding is constant adjustment. If poop is runny, add more bone, if it is too hard and he is struggling too much, add a bit more liver. You'll be adjusting every week or possibly every day as he grows up, and you'll get used to it quickly.