Maybe not a pillow when you're outside the house, but something along the same lines that you can take out aggression on. Something like a hacky-sack, which you can throw but isn't likely to hurt anyone or anything.

[Stress balls would also be good, but probably less likely to find. And okay, well, with that question he supposes he's going to have to just say it.]

Normally, managing anger issues effectively in the long term requires handling the underlying issue causing it. Even if your emotions are being amplified, as long as they're not coming from somewhere else entirely this method should still work.

[But, probably obviously, this is somewhat new territory.]