Actually, the boar only triggers the extra cutscene where Astarion offers to stand watch at night, not the bite scene.
Usually, unless something pushes back his scenes (scenes with Urge for example), he'll come biting the next night after he's gone hunting.
It's unlikely you'll be able to push back his bite-recognition scene for long, unless you avoid a long rest (usually 3-4 nights). He's hungry, that's all the cards on the table right away))
So, if you see an elf sneaking into the forest in the night, he'll come to you at the next long rest.
