It was practice, of course it wasn't bullying. Jack probably wouldn't even judge if it was. He's just feeling restless and unhappy, and one way he knows of to deal with that is to fight somebody. He's not the best fighter out there-- he's a prince, he's got bodyguards for that sort of thing-- but he was the commander of his unit in the army. He's got some training and experience, at least.

So he takes a moment to look over Riou's stance, half-crouched on the balls of his feet in his expensive (and probably now ruined) palace shoes, then lunges low, aiming to avoid being grappled. Riou is bigger and stronger-looking than he is, so getting him off-balance and moving quickly is the goal here.