Prepare yourself - boys get boners. Even baby boys. My sweet, precious baby, who is 16 weeks old today (sigh) get boners. When it comes to H, I prefer to call them "diaper tents," I don't feel as creepy talking about it then.
I'll never forget the first time I dealt with it. He was two months old, I take his diaper off, and there it is. All proud and smug. Of course, I didn't believe it. But there I go, try pushing it down to clean him and it just pops back up, no problem. Now, if you come across this - duck and cover because diaper tents are not just any boners, they are pee boners. I have learned this the hard way. His pee shot straight up into the air down. Down all over him. Down the changing pad. Down the walls. Down me.
It's quite the shock when it first happens but you quickly get used to it. And thank your son, he has given you the gift of easy jokes.