He won't be deprived of nutrients in the amount of time it takes to transition. Right now, you want to go protein by protein and not add other stuff to the mix. That makes digestive problems more likely. It would take months of feeding only one protein to be lacking nutrients. Once you have gotten through a couple of proteins without issues involving digestion, you can slowly add beef liver in very small amounts if you want.
If you go with turkey next, you can use necks or thighs. I find drumsticks are too dense for mine, and after a couple broken teeth I don't use them anymore.
If you go with pork, ribs and/or riblets are fine.
Either way, you want to get poops as firm as possible before moving on, so you don't have issues carried over to the next protein.