Rule outs for chronic (months old) mass: tumor (benign or cancer), abscess, chronic irritation (non healing wound due to repeated trauma, continuous pecking), a cyst (fluid filled tumor or mass, again can be cancer or benign), fungal infection, congenital defect (present since hatch). If it was cancer, I’d assume he’d be dead by now. Abscess, fungal infection, and non healing wounds can be treated medically (lancing, medication, separation until healed). Benign tumors or cysts can be removed surgically (if you want to do surgery on a quail!) but if the bird is eating/active and the pen mates aren’t bothering it, benign neglect is also an option.