My 4 yr old nephew has had a bruise on his cheek for 3 months, it's greenish in color, and there's a small lump underneath the bruise. any ideas?

Photo?: Impossible to diagnose without direct visualization. It "sounds" like a bad subcutaneous bruise or hematoma which is old blood under the skin trying to resolved: usually the old black, blue, yellow ,green phase depending on the injury- have him seen and evaluated-ASAP.

Lump: Was there an initial trauma? If so, it may be an area of fat injury but that should have resolved by now. Have your doctor look at it or send a picture to help with the diagnosis. All the best in getting this sorted out.