A friend of mine's DS had what they called a "fat pack" that made his penis seem quite small unless it was erect. Does he have sort of a fatty bit at the base of his penis, but on his body not the penis itself? If you give it a gentle push his penis might appear to come out a bit... Kind of hard to explain. What she was told was that it was just a build up of fat that made the penis disappear a little bit & with time it would go away. By the time he was two it was gone & it all looked normal.