It's very easy to read his emotions based off the flicker of his eyes:

Moving upwards and breaking off (like the tops of fire): angry, determined, passionate

Gentler movements, from all directions (like the sun): happy, calm, relaxed, feeling safe

Irregular pulsing quickly with smaller movements: nervous, anxious, scared, worried

Irregular pulsing slowly with bigger movements: sad, depressed, empty

Flickering playfully from all angles: smug, plotting something, you should probably hide your children

Violently flickering like a bonfire: only happens when he's on a killing spree, or when he's really angry. Basically when you should run away

Becomes a glow with an inconsistent flicker every now and then: curious, wondering, not understanding, confused

And with any possible combination.