Allow him outside. Give him room to express his grief. Keeping him inside the Pokeball is essentially like bottling up the emotions he's feeling.

[His tone isn't an accusing one; it's gentle. This person's done nothing wrong so there's no reason to make him feel like that.]

It sounds to me like he may need a good bout of screaming. And when that's done you could try speaking with him. Gentle tones--give him space but make it clear you want to be there for him.