I say it over and over, but it usually falls on deaf ears.
Sugar. Drastically cut back, and be aware of sugar in bread, etc. My son gets migraines from sugar, and it wasn't until he was 13 that he could describe what he felt well enough for it to be identified. Even if he's not getting migraines, it can make him angry and aggressive.
In addition, if he has other food allergies/intolerances, he could be experiencing adrenaline reactions (fight or flight).
Keep a food/mood diary to see if you can see trends, then remove possible trouble foods to see if there's improvement.