How long will you be away for? If it's only a few days I would just beef the betta up beforehand and leave him to fast while you're away, that way your water will stay clean. Those weekend / holiday blocks can foul up a tank pretty quick.
If you feel you have no choice (and nobody who could be trusted to feed him while you are away) then you should try to find one that isn't composed largely of plaster-of-paris. Tetra do a large one that is gel-based instead, I chopped one of these into quarters and used for my guppies with no ill-effects.
But I would check to see whether your betta will eat it first, mine only eats food that floats on the top of the water (except for bloodworm, he'll chase that as it falls). Maybe try him on one the week before you go?
