Should I tell him about my feelings?

I (27F) have been in an open relationship with my bf of four years now. We have each dated other people and it has never interfered with our relationship before. Four months ago I started dating someone I had met at uni and vaguely known for a while. I made it clear at the beginning that I had a bf and I was only around for casual fun. Me and this guy get on really well, we have great conversations and I love spending time with him. We message most days and see each other once a week. He also said at the beginning that I should tell him if our involvement threatens my relationship with my bf. I realised I have deeper feelings for him a few weeks ago and have been struggling with it since. He writes for the student paper and recently gifted me the new issue. When I read it I found a column he had written about a deep connection he formed with someone that he was very grateful for - I know it's about me as he included a reference to the day we first started seeing each other. I don't know what to do. On the one hand I want to be honest with him and tell him about my feelings, as it's the right thing to do and maybe he feels something for me too? But I'm also worried that the revelation might push him away / make him end our involvement if he doesn't feel the same. I have spoken about this with my bf and he says I should definitely tell him about my feelings because then at least I'll know where I'm at. While this is absolutely right I don't want to lose this other guy. He's such a wonderful, passionate person, and while that column is heartfelt and beautiful it might just be him appreciating the connection without feeling the same for me as I feel for him. If I don't tell him I might be able to keep him around a little longer / spend more time with him before the inevitable end of the relationship. What should I do?