If you intend on being with him for the long-term, which it seems like you might be considering you’ve already committed over a year, you need to tell him! Just explain your sensitivity to discussing it and I don’t think he’ll be mad. Honestly, he might already know and just didn’t want to mention it until you do. If you want to be with the person long term, you shouldn’t hide it- for his sake and yours! You’ll feel more at ease with yourself if he just knows.