It could be anything from a
broken blood vessel (which is pretty much a hickey) or an STD. Any type of trauma to the penis can cause blood vessels to break. Is it tender to the touch? Does it feel warm? If so, it's probably a blood vessel broken. It could have been caused by a number of things, your fingernail digging into his penis, him pushing too hard...ect...I wouldn't worry.
But yes, a hickey can show up anywhere where there are blood vessels...pretty much your entire body.