What did he mean by this?

He can tell you where Moltres is if you give him 99 Burn Heals. Trust me, my uncle works for Nintendo.

He is the fire type gym leader, and as such, his Pokemon can inflict the burn status effect on your Pokemon, meaning a burn heal would be useful.

>>27508017
He is warning you against the STAB Scald that all his Pokémon havehave, as he is Water Gym Leader.

>>27508017
He felt the Bern.

>>27508017
He is hinting at the Moon Ball in the next game, which has a higher catch rate when used on Pokemon that evolve when a Burn Heal is used on them.

>>27508017
He means you can use a Burn Heal to unlock the truck near the S.S. Anne and obtain Mew, but only after you also find the keys to the door left on at one of the Game Corner's slot machines.

>>27508017
He was feeling pain from a recent burn he got while cooking and was demanding medicine for it.