Bunny - Strange change in eating behavior?

Recently my senior rabbit has stopped eating his usual pellets, as well as his hay. He will still eat things like kale, cereal, carrots. I don't understand why he might be changing his eating preferences. He still chews wood toys around the house - more so now than ever before. He doesn't seem to be in pain, and is still excited around feeding time, but then won't eat the pellets. Energy levels are still fine. I am concerned about him losing weight. Any ideas as to what could be wrong?