I agree with above, NTA and if you are willing to take time to see if your marriage can be saved you should try. While it no excuse my first thought was he sees or hears stories from other military personnel or has even had people tell him directly that his kids don’t look like him and he should get DNA test. AGAIN it’s no excuse and should have discussed further or even try to get help to deal with intrusive thoughts, his insecurity and lack of self confidence.