My 8 year old son has a lump under his left nipple. I have researched it and fear cancer. Could it be something else?

May not be cancer... As a pediatrician for 30 years, I have yet to find breast cancer in a preteen boy or girl. A more probable likelihood is a dormant breast bud or a subareolar lymph node, both of which occur often in my practice. Of importance is there any bleeding or oozing of pus or fluid from the nipple? Is the skin surrounding the lump red or hot? Is it painful? For peace of mind schedule a pediatric exam.
Highly unlikely. Your 8 yo son is at very very low risk for breast cancer. It is most likely a benign (non cancerous) mass. See your pediatrician for details. A biopsy might be in order but the likelihood of cancer is extraordinarily low.