I would NOT use a feeder block unless I was standing directly next to the tank to see if he ate it. And I would then immediately remove it again.
I once used a feeder block for a weekend. When I got home, it was sitting at the bottom of the tank, encased in a cloudy goo of decomposing food and material. My Betta was curled in her hammock, not moving and completely clamped up.... I did an immediate 100% water change. It took her several days to recover.
After that, I looked into feeder blocks more. I found that they don't usually contain 'good nutrition.' And they contain other ingredients that can foul the water quickly if the fish doesn't eat them.
Some people have success with them. But Bettas often want to eat at the surface, not the bottom. And since they're mostly junk food and non-food fillers, I would suggest not bothering with them at all.
I feel that doing frequent water changes is the most important thing you can do for your guy right now. Once he is in clean, warm water, he'll probably feel better and regain his appetite.
If you want to pick up some new food, look for pellets that contain whole fish as the first three ingredients. A lot of people on this forum highly recommend two brands of pellets:
- New Life Spectrum (NLS) pellets
- Omega One Betta Buffet pellets