Throw a ghost shrimp in there, he will eat it in time . . .
Slow release blocks are a big no-no, they will compromise the water quality. Feed him well before you leave, change his water, then unless you have an automatic feeder he should be all right.
Bettas are rather resilient. Ages ago my little brother had a betta in the worst little bowl and I could easily have saved it, I'm sure all it needed was a water change. . . that was before I kept fish. Food was also scarcely put into the tank.
Auto-feeders are the best, although one would need to spend upwards of $40 for a good one. I went through 2 crappy ones thinking they would do, returned both. Bought a digital auto feeder for $50.00 and never looked back - too reliable.