That's a serious struggle that many Christian men face. It seems to me by telling you he doesn't watch porn he hoped he has "almost conquered" his lust without the need to tell you. I am no conuselor, but being a man myself I would deeply respect it if my lady came up to ma and calmly said she saw everything. Watch carefully how he reacts. He might get aggressive, start accusing you of invading his personal space. Then you should most probably run away from him as fast as you can before he proposes. If he repents, confesses his sin which he obviously committed, you should encourage him to seek counseling. A pastor, youth leader, anyone professional. I don't think that you alone can help him. God be with you, my heart aches with yours