I'd also like to point out that if he wants to keep his team safe, his best grinding options are to grind on wild Pokemon that are like 10+ levels under him. The Grand Hall trainers all come with an exponentially higher risk of killing off members so he's best off grinding on wild Pokemon to not risk losing anything, and trust me that's bloody TEDIOUS. So yeah, it makes absolute sense that he doesn't grind much, it's just not very enjoyable and he hasn't really been in need of it as long as Belly has been around just soloing fights.