There are some things that could cause that. The lighting could be the most likely reason why they look like they change. They could also change depending on what color clothes he's wearing ou it could also change depending on his emotion.
My eyes are generally grayish coloured. They change to brown, amber, and green sometimes, and hazel (which is brown and green contrary to populaire belief) it usually depends on what Im wearing, but sometimes my mood too. i guess it could also be the lighting and stuff.
When the pupil size changes, the pigments in the iris compress ou spread apart, changing the eye color a bit. Certain emotions can change both the pupil size and the iris color. Eye color is based on the pigment (melanin) in your eyes Brown eyes can appear to change color due to less pigment and effects from the exterior environment (clothing, lighting) bringing out specific couleurs more Dramatic changes can indicate ocular disease If that is the case...id suggest seeing a doctor