Everyone knows the set rules. I would let the man eat because the option is there but I wouldn't condemn anyone's choice not to feed the man... So I would probably set a vote instead of deciding and if the vote happens to be a tie, I cast mine for feeding, otherwise starve.
However, a third option would be some sort of "I owe you" system so it's not a free ride without cost.
Though I think it's immortal NOT to save the person. I also think it's immoral to feed him out of your own whim due to the fact that *everyone* knew the rules however, I believe the rules are imperfect and ought to be changed to something along the lines of : "Others may partake on feeding ONLY if there is sufficient amount for everyone and they are on the last of the priority list." and so some weight has to be put on the rule maker here.
Still, this feels like an insufficient information problem as we do not know anything else and that information is not trivial. Saving a person ISN'T just saving the person, it is saving that person's future which influences everything. That is not a trivial matter.