What causes a swollen lymph node in the groin for men?

My boyfriend is experiencing a swollen lymph node in the groin that keeps going up and down and when it swells up it causes him pain. My first thought was infection. I had strep when this first popped up for him and I really thought that was his bodies way of dealing with it but he never got sick. I then thought well maybe I have some kind of infection like yeast and I don’t know and I gave it to him? (I have had a yeast infection with no symptoms and only found out when I got a Pap smear)I have no idea how a man’s body works when it comes to stuff like that, how their bodies show symptoms I have no idea but I know yeast can be passed to men during sex. I knew it wouldn’t be anything STD wise because I was tested right before I got with him, because my ex before him was a serial cheater and I didn’t have anything but i wanted to be sure I didn’t pass a yeast infection to him. I have health insurance and he doesn’t, so I went to the gynecologist, asked them to test for infections. I was tested for every infection under the sun. No STD, no yeast or BV.

Soo I don’t understand what on earth could be causing this for him. It really hurts him when it swells, he’s currently sleeping with a heating pad.

I’m really worried about him and told him he needs to see a doctor but that’s going to cost a lot of money with no insurance.

I tried asking this on Reddit because there’s men there but TBH I don’t even know what sub to post it on, I found one but my post was removed for “being too long.”

Sorry if this is sloppy I have a horrible migraine