Thoughts on threesomes??

Hello everybody, I've been in a relationship with my boyfriend for a bit more then a year, everything is going well but he kind of suggested that he wants a threesome with another guy (which first I was kind of bummed about, but after some thought I am okay with and would like to participate in). But I am afraid that he will no longer feal attracted to me sexualy when he has sex with this other guy, and that I end up getting nervous and not be able to to anything with him and that other guy.... I guess that most of my insecurities come from the time that he said he was kind of bored with my penis. I've never been much of a top, even if I do want to hear never let's me, we barely did anything that involves him even touching my dick how is he bored with it?? So I guess my question is, what should I do? Should I tell him that I am okay with doing a threesome with other people but that I am worried that I won't be able to be intimate with this other guy? Should I even go on with the threesome? Also if you have any advice about threesomes in general or comments or whatever, please submit then bellow because tbh I need all the help I can get. Thanks so much ☺️