My 11 month old son has a very hard bump under the skin on the underside of the shaft of his penis. it's as big as 3 pinheads and hard like plastic.

Bump on Penis: Common lumps on the penis can be from inclusion cysts and scarring from minor trauma (such as circumcison). The lump under the skin may also be either a sebaceous or inclusion cyst that was trapped under the skin by the circumcision. Almost all such lesions as you describe are harmless but if you want a more exact diagnosis, it is best to see a urologist.