You should feed him Purina on the day that you no longer want him.................

I really appreciate that you are willing to put the effort towards cooking food for your dogs and feeding only homemade food to your dog but you have to be very careful about their diet. Most veterinarian will tell you about the important connection between nutrition and health of your dog. So it is very important to have the right balance in their food unless your pet will face some health issues. Most of the dog foods are well balanced. You can always give your dog canned food after carefully checking the ingredient grade of the food.

Why do you want to change the food of your dog if it is working well for him and he has no problem with it?

The saying "if it ain't broke, why fix it" applies here. We all try to do best by our dogs and for some that means feeding raw, for others it is feeding a dry complete food and, as you have found with your own dog, feeding a home cooked diet has worked well.