It's really very hard to tell you over the internet what this lump could be...but since you have this lump for so long and it didn't grow or change in appearance,I doubt that it could be breast cancer.
Breast cancer is quite rare in men,but it can happen especially when they get older.
The best advice I could give you is to have this lump checked out by your doctor.Maybe running some tests will be recommended to find out the true nature of this lump.
Please go see your doctor or a dermatologist for an accurate diagnosis.
Take care and best wishes...