Welcome! I agree about trying the hikari pellets. All of mine eat them. Though Whiskey wouldn't eat for the first couple days. They also sell a mini pellet which is good for smaller bettas if that seems to be a problem for him.
You could try frozen bloodworms too and frozen mysis shrimp. Most pet stores will have them in the freezer. At least then they're not dehydrated. I use tweezers to feed these to all my fish occasionally (well, my puffer gets them every day).
Just watch for bloating. His stomach is just behind his gills so if he starts to look bloated there, fast him until it goes down. Sometimes bloodworms can cause constipation.