My son is 11 year old. His penis is very small (approx 1 inch). Yesterday (10 March '09) while he was playing, he noticed the penis 'gone inside'. He was really worried and started pulling it out. Why does this happen?
One of the worst things a boy can do is to compare himself to others (even brothers) to assess his development (although we all do it). "normal" can be many things and different people develop at different ages and different rates.
It's not unusual for a 9 year old to have not begun puberty. In fact, 9 is considered "early normal". If you do a search on the term "Tanner Scale" you'll find a lot of information that you can share with your son. The Tanner Scale was developed to ***** pubertal development. It considers both genital and pubic hair development in boys.
There are circumcisions and then there are circumcisions. Depending on the technique, sometimes some excess skin is left. When this happens the penis can retract back under it a little. This can look very different from a circumcision where the complete foreskin is removed and the glans (head) is always completely exposed. This is especially true when the penis is fully retracted, such as when it's cold, it's been compressed by his clothes for a while, etc.
Another factor that can affect how the size appears is weight. If he has a few extra pounds on him the fat pad at the base of his penis can make it look shorter. (Think of a stick in the sand and then add some more sand around it. It will appear to be shorter but it's really not). If your 6 year old is thin and your 9 year old a little heavier this will give the illusion that his is shorter.
As to why his penis doesn't "grow" when he has to pee, that can be something that he observed on one or two occasions. Sometimes a full bladder presses on the nerves of the penis causing an erection. This might be what he saw with his brother. Or, his brother might be in the habit of pulling on his penis, fully stretching it, before he pees. (many males including adult men have this habit.
The important thing to stress to him is that he is normal for him. Puberty should be just around the corner for him and changes will start taking place at lightning speed. As long as his penis functions normally (i.e., he gets regular erections) he has nothing to worry about. I suggest that you ease his mind by helping him learn about his body. As I mentioned earlier, do a search for the Tanner Scale and let him participate. That way you can both learn at the same time. Another good source is a book called "What's Going On Down There? For Boys." It's written for someone his age and is available in most bookstores. A good understanding of how broad the definition of "normal" is and knowing what to expect during puberty will go a long way toward his peace of mind