I think you’re being selfish and self absorbed. You should back off for a bit and not bug him. When there is something I don’t want to talk about, it’s because it was rather traumatic and still really hurts. As I feel closer to someone, I open up little by little as I trust that that person isn’t going to hurt me. It’s difficult to feel vulnerable. Don’t worry about whatever happened way back. Just focus on the present, and create a loving wonderful life. Everybody makes mistakes and has done things they regret, to keep throwing it in his face is just mean.
Say a couple of ‘I statements’ in a loving way, like ‘Sometimes I’m concerned if God sees our marriage as valid since I’m not sure if your first marriage was annulled or not’ and ‘I love you so much and I want us both to go to heaven, so I hope he does’ then drop it, and talk about something loving and non-confrontational. As he feels closer to you, he’ll come around.
Perhaps his first wife and child died, in which case no annulment is necessary.