unusual hard spot

I have a 6 month old yorkie. He loves his belly scratched and I noticed an extrmely hard spot at the bottom of his penis right above his testacles. It's almost rock hard. I'm worried...any clue what it might be?? hasnt had his normal appetite and has vomited a few times within the past 2 weeks but I figure it was because he eats everything he sees becuase he hasnt seemed ill.