My Boyfriend Is 50 And Has Constantly Hard Nipples. They

My boyfriend is 50 and has constantly hard nipples. They get harder when sexually stimulated, but they are pretty much fully erect already. I like them, but evidently growing up, it was an embarrassing topic. He is fairly healthy and he is a guitar player. We are both fairly intelligent, and even with searching the internet, cannot find a serious answer to this perplexing question...