In my opinion, your guinea pig has grown tired of his hay. It sounds like he still has an appetite besides hay, so I think it just the hay. Kaytee is horrible. The hay itself is actually very hard and SHARP, which can cause injury. I actually have scratches on my hands from handling their Timothy Hay.
For the past year and a half or so, my sister and I have been loyal Oxbow customers. Oxbow is a great step-up from Kaytee! Their Orchard grass is my girls' favorite. Recently, though, the weather hasn't been the best and so Oxbow's hay has suffered. We decided it was time to order Kleenmama's.
On Saturday, we received our shipment of hay and pellets and I, myself, am in love with this hay! We ordered 10 pounds of 3rd cut Bluegrass. It is so, so very soft and moist and it smells wonderful. Our original three girls; Freya, Haylee, and Villie, eat it constantly and we have to refill their cage so much more often than we ever did with Oxbow. The new girls are a little slower to eating the hay, probably because they are still scared, but they have eaten a great amount as well.
I truly think that a better quality hay with improve his appetite. However, it is never a bad idea to see a vet just to be sure.