The best way for him to feel OK would be a visit to his doctor for an evaluation...

It sounds like you son is developing a case of Pubertal Gynecomastia. See a doctor and keep careful watch over this condition. The condition is harmless, however in many cases these lumps grow into small breasts require expensive surgical removal, and can invite unwanted ridicule from peers which can destroy your child's self esteem and confidence. Your doctor will most likely tell you it will go away within 3 months. If it does not, talk to your doctor about estrogen inhibiting drugs such as Tamoxifen, Raloxifene, and Anastrozole which should help with the breasts. However the sooner you start him on these drugs the better. The more you wait, the higher the chance they are going to be needing surgical removal.