Rabbit doesn't actually want to order the kid around, mostly cause he remembers what being a pirate cadet was like. Realisticly, he'd punch the Captain of the Guard in the face for entertaining the thought of bringing a kid to a battlefield now that the Galactic Federation has informed him (numerous times) that children are not to be considered enemy combatants until verified as such. Rabbit doesn't see Jeremy as a combatant, just the handler for his charmed armada of monsters that don't handle ice or water well, and since there's a plane of ice and an ocean between us and the ship, well shit...
This combat will potentially change Rabbit's view of Jeremy, considering his Ivysaur just took out a pirate.
As for what to do to help... Aid Another actions on the siege engineers to perfect their aim and take out some of the deck crew. Make Intimidate checks to bolster allies (The reverse of Demoralize, so actually probably diplomacy or just "We can do it!" speeches. And everyone said bards were useless
). Jeremy can start to see if Pokemon potions can heal people, elves, and whatever the hell Kurt is. Barring that, he can declare injured combatants and enemy fighters as evolved pokemon and try to capture them in pokeballs.