Well, technically, DragonBreath is a Special move, so it would be on a different tree from the moves he has, but since he's already learned moves that are higher-level than the previous Special move (SonicBoom, which is a starting move, anyway), I'd just take for granted that he knows SonicBoom. I'm still deciding whether to just allow people to have their pokémon know<em>all</em>moves they learn before level 20 or whatever, and to be able to use any moves they know whenever they want.