Husband Adopted/ Struggling with Search

My husband was adopted at birth- he was 3 days old when he went home with his adoptee parents. He was told at 13 and he told me he made himself a promise to not look for his birth family out of respect for his adoptee parents. I believe he is struggling with depression over this decision because he wants to look but he feels like he would betray his adoptee family. This has come up several times and we've been together 8 years now and its a subject he's not comfortable talking about at all and never brings up. I feel uncomfortable bringing it up but I'm not sure what to do since I see as of recently he looks lost. I told him if he is struggling so much maybe his adoptee family will understand his need to look. I'm lost at what I should do. He's gone back and forth about just me looking for his birth family but I have no information and we doesn't want his adoptee family to know. Anyone? Help? Thanks!