Should I be worried about this lump by my son's chest? It's only on one side of his chest not the…

Should I be worried about this lump by my son's chest? It's only on one side of his chest not the other and I do I know that he gets hormones through the breast milk and I can cause it this is about an inch all around in size and on just one side. my mom and my grandmother say I should probably call the doctor? has anyone ever seen this before though?