My fiance has been concerned the past few days about the tip of his penis vibrating and we were wondering what it could possibly be.

Fasiculations?: "twitching" from fasiculations can occur on the skin, but "Vibrating" is an odd symptom, or at least description of something that happens on the tip of the penis. If very concerned, see a dr. when this happens and let the dr. see it and examine you to get a likely answer. Most likely a benign occurrence, and you would be a rare woman with a fiance who has a vibrating penis. Best wishes.

Nerve injury: Hi Jeffrey, Nerve injuries can occur from prolonged sitting or activities such as cycling. This symptom has been associated with Piriformis Muscle Syndrome but also can be associated with prostatitis, an infection of the prostate gland. Without any fever, chills or urinary symptoms my diagnosis would be the nerve injury. Treatment is stretching and stopping the causal activity.