If he is having normal stools, you can start to add in other meats, one protein at a time if you want. You could use the mince you have if you like. Just add a small amount, maybe a teaspoon, to the regular chicken and see how he does.
I wouldn't recommend a whole meal of a new protein all at once. Build up to it gradually. If he just eats the mince, you may have to dice up the chicken and mix them together. You can gradually decrease the chicken until he's eating the new protein by itself.
Keep in mind that the bones are important for calcium/phosphorus as well as keeping the stools firm. So you can bash them up with a hammer and mix in with your mince if he won't eat them.
"Having heard all of this, you may choose to look the other way.....
but you can never say again that you did not know." William Wilberforce 1759-1833