Putting some flamazine on it or diluted chlorhexidine should help until the vet appointment. Flamazine is antibacterial cream so should settle the inflammatory response down a little and decrease the amount of tissue damage and if the lesion is not bacterial the cream will still prevent a secondary bacterial infection establishing, will probably cost about $12 from the chemist
Use it sparingly though, as with any drug there can be side effects if you put too much on.
Looks pretty uncomfortable for the poor fella good luck mate.