Just found out I have a micropenis! It´s 1.5" stretched... I am 21. I can tell you that it´s not that bad being that small I can totally still have normal sex so don't worry about your son

The short answer here would be to have him evaluated by a physician to see if there is an abnormality. Since he is 12, you may want to take him to his pediatrician and talk with the doctor privately before the examination, so he isn't overly embarrassed. He may need evaluated by an endocrinologist or a urologist, depending on if he does in fact have a medical issue. If you are concerned about it enough to post it on this board, it may be worth getting a second opinion if your doctor initially brushes it off without hearing out your concerns. The biggest issue you have is sensitizing your 12 year old into thinking there is something wrong with him. So it needs to be handled delicately. Has this ever been brought up when he has had a well child check? Doctors usually examine the penis and testicles during those times. You have seen all 3 of your boys nude for a while now, but a different perspective on this may be that boys penises usually don't grow a lot from age 5 until the beginning of puberty. So comparing a 12 year old and a 4 year old isn't a fair comparison. Put a different way, the same size penis on a man who is 5'7 and 135 pounds is much different that a man who is 6'4 and 220 pounds--though they both may be adult normal size. So your younger sons may have the perception that their penises are unusually large compared to a 12 year old who is about to begin puberty. I would trust your instinct on this one, however. Everyone's body's respond different to hormones, so your son's development may need to be tracked as he progesses through puberty. A smaller than average penis before puberty doesn't mean he won't have a functioning adult size penis. Good luck.