It’s difficult for me to know how to advise you on what to do because I don’t know enough about the emotional state of your boyfriend. Let me only offer you some thoughts: You’re probably correct that he is embarrassed and scared. You’ve said that you do love him and you’re prepared to be supportive. So I find myself wondering, which is worse: To keep his secret to save him embarrassment and watch him suffer and get maybe more desperate — or to tell him what you know and offer your heartfelt support, love and help? I also wonder if subconsciously he left that document on the counter where you could see it. That way, he didn’t have to face you to tell you.
Life can be hard. One of the most important reasons people become partners is so they don’t have to be alone — in good times and in bad ones. There is a good reason that marriage vows include “for richer or poorer, in sickness and in health.”
Perhaps it would be helpful to remind your boyfriend that you are in this relationship for the long haul and you take the sentiment seriously, whether or not you are married. If you can find a way to talk to him without shaming or blaming, it may be a relief to him to know you are on his team. But you know him. I don’t. Be mindful of timing and of his state as you decide how to approach him.
I wish you well.