you buy a drink from the vending machine on the roof of the celadon pokemart thing, the big 1. then if you go through one of the gates he should say 'jeez im parched' and then he should take your drink
in heartgold and soulsilver, {you dont have to defeat vermillion gym} just go to the route above vermillion and battle trainers or walk through tall grass you keep heading north until you just walk...