Please start with his PCP. Many boys and men have breast cancer. Not saying this is what he has. Go to breastcancer.org and join the discussion board there. There is a forum just for men that could possibly give you more info. Hope this helps.
One of my closest friends had a similar problem with her 16 yr old twin son. He also had a lump, they saw a breast surgeon privately and it was deemed to be fatty tissue. He is now a healthy 27 yr old and doing great in health and his carerr. For peace of mind, I would get it checked out.
I am posting this comment here since I found this thread by searching for 'male breast cancer' on the forum. I hope it helps someone.
I truly hope the 16 year old fellow involved found that his lump was not cancerous. And it reminds me of when I was 17 and discovered a similar lump. I recall embarassment and relief when our family doctor lanced the lump and squeezed out what I thought was a boil. ( I have later been told that it might have been something called a serbaceous cyst, but we will never know for sure now.)
Fast forward 45 years to my current age of 62. Until now never thought much about that teenage lump . But now I have IDC in the same breast at the same location of that earlier cyst. My surgeon says that the two may be linked in some way since those cysts can turn cancerous given enough time.
My advice is that boys should be told to be aware of their breasts and any changes they notice--- their whole lives. Tell them there is nothing 'unmanly' with self exams and to inform their doctors if they ever had a cyst before. It is easy, painless and could save their lives in the future.