Dear Gary,
If he will not eat any canned, you need to switch him to a dry food that is high quality and contains no fillers. You can try nutro, felidae, wellness, science diet, etc.
In addition, give him a half teaspoon of mineral oil three times a week with a dropper. you can also get a product called lactulose from your vet.
It is most likely the ingredients in the purina are causing his constipation.
Let me know if you would like to see some websites for better food choices for him.
best wishes,
Terri
__________________
Dedicated to cats and the people who love them!
If my answer was helpful please press ACCEPT and leave feedback. Thank you.