Hi and welcome to Binky Bunny. It sounds like a vet visit would be good for your bunny. It’s natural that a bunny might slow down at that age, but he could have arthritis or some other problem that is making him less active and eat less. If a bunny has pain or discomfort from arthritis, he probably won’t feel like eating as much. I have known rabbits that benefited a lot from small doses of pain medication, like Metacam, when they were older. He also should have his teeth checked. Dwarf rabbits can especially have trouble with their teeth. And if he is having discharge from his eyes, that should be checked too. He could have an uppper respiratory problem or that could also be a sign that he might have teeth problems.
If you have never tried or haven’t tried lately, different brands and varieties of hay, you could try some now. Maybe you could find ones that he likes better. Some rabbits prefer softer hay, such as second or third cutting of Timothy. American Pet Diner has nice second cutting hay – Timothy Gold. You could also sprinkle a little alfalfa hay on top of his normal hay. It’s very fragrant and a favorite of many rabbits.