Hi Sara, I'm sorry your guy has lost his appetite. Here are some tips that might help:
One thing you might try doing is increasing the water changes to one 25% and one 50% a week. 50% is fine but I'm not sure if it's somehow putting his appetite off. You can try the increased water changes and see if that perks his appetite up.
You can also try soaking his pellets/food in fresh garlic juice. You can use fresh squeezed garlic or you can dissolve some garlic powder - garlic only, no salt - in tank water. Garlic can stimulate the appetite.
You can try holding a mirror up to him right before you feed him or right after you've fed him. Sometimes if a betta thinks he has competitition for food his survival instinct will kick in and he'll eat because he doesn't want the "other" betta to get the food.
If you feed him frozen bloodworms, you can use tweezers to pull the worm through the water to make it look alive. This might stimulate your guy's hunting instinct and he may think the food is prey to be eaten.
I hope one of these tips helps you.