Any advice on relationship, please?

I've been dating this guy for two months now. I love him, he says he loves me too and his actions show that he means it.The thing is that, when I met him, he was in an open relationship with a guy he hasn't seen for the last 4 months, even though they are still in contact online. At first I didn't really mind it because I wasn't in love with him , but now it bothers me a lot , because officially they're still together.I've been clear from the start that I wouldn't want an open relationship, so I confronted him about this and he told I'm the one that his loves and that his other boyfriend knows that he's seeing someone else.I don't like this situation, firstly because he's boyfriend thinks it's just a part of their open relationship. Second because it emplies that he's cheating on his boyfriend. And more importantly, for me, I keep wondering why he didn't break up with yet, since he says he loves me.I'm so jealous 😣😣