Husband talking to another woman

My husband and I have been married a year. We hardly ever argue or fight. I feel we have a very good relationship. There’s been this girl messaging him on Facebook from his past. The only reason I originally knew she had messaged him was because her name popped up on his lock screen a few times. I thought it was innocent conversations. Well last night I had noticed they’d been talking back and forth so I went through and read the messages maybe I shouldn’t have but I did. She mentioned numerous times not like me and my husbands responses were nice. Nothing alarming. So I didn’t look any farther. Well I couldn’t shake the feeling that there was more to it than that. So today I looked through his messages again. She’s sent him nudes. They have messages going back to last year. And he’s said some things that he shouldn’t have been saying especially considering he is married... I don’t know what to do. This is the man of my dreams. We are trying to have a baby. Do I confront him? How do I go about it? I know he’ll be pissed once he finds out I read his messages. Any advice would be wonderful.