I think the reason for your cat's appearance in loss of apetite is simple - purina cat food is loaded with junk. It's an addictive food. They put additives and so much junk in it like salt (why does a cat need salt??) and "animal fat" (which animals?? lol... and often this means sprayed by oil/fat/greese..). This all causes your cat to be addicted to their food. The reason they do this is because otherwise, your cat wouldn't bother with it. So, that's the reason I think... but the solution isn't as simple.
If it's a kibble try some water in it.. try just leaving it out all day for him to grab as he wishes. Another thing too.... higher quality foods require them to eat less of it. So, if you're feeding the same amount as you did with purina, you're very likely overfeeding, and he's just full. If he isn't eating it all, that's not good either.
I know it's been a while since you posted this but thought since I was in the nieghbourhood I could try to explain why he's all of a sudden showing signs of annorexia. I went through this with my cat too. Unfortunately, (you'll see from my other post), I can't really provide you the same solutions I have resorted to as my cat is severely allergic to chicken. On that same token though, try a different protein source. If you're feeding him chicken, check out a lamb diet, or venison (though venison is always much more expensive).