Specific medical or surgical therapy is usually not indicated in this age group since it most commonly is related to the hormonal changes found with puberty and tends to go away on its own with time. Of course, teenagers are also very much aware of their body image and have strong desires to fit in with their peers. Sensitivity and reassurance are therefore important to keep in mind.
Since your son apparently is overweight and I cannot tell by what you have written how far along in puberty he is, I am uncertain whether he has gynecomastia by the definition above. What you can see now with your son may very likely just be fatty tissue more than true breast tissue. A good history and physical exam, including feeling of the breast area by a health professional, may help distinguish between the two. Improving the quality of what your son eats and decreasing the quantity along with a good exercise program may be all he needs.
Interestingly, as your husband suggests, there are other causes for enlargement of the breast in this age group. Underlying diseases -- gland problems, drugs, tumors -- much less commonly cause gynecomastia in teenaged boys. For example, a significant ingestion over time of birth control pills, which contain specific hormones like estrogen, could be responsible for gynecomastia. However, I don't think a one time ingestion of birth control pills by your son at a young age should be related to what you are describing now seven years later.