Is it possible to just accept that maybe your husband won't like it, but you're doing what's right for your son and that's what matters? What is it that you're afraid your husband will do when he finds out, besides fuss? Part of the beauty of being separated is that you don't have to worry as much about his fussing--you're free to do what feels right to you!
Is it possible that your husband has a point that you are engaged in a "come here, go away" pattern with him? You've mentioned dating other people recently but still seem to be keeping him pretty close. Maybe you should try out actually being separated? Like maybe a whole month or two with no sleepovers or physical intimacy? It seems like you want that on some level.