maybe you didn't see my answer but i did already send it to you. here it is again:
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There are adults who still do urinate in their pants for various reasons. For some it started as a comfort or coping mechanism in childhood (like your boyfriend) and since he never told anyone, or tried to stop he continued this behavior. And now after all of these decades, it has become a way of life.
I would ask him, if the wetting is in any way, related to erotic or sexual pleasure for him? If so, this could be part of a sexual fetish, called infantilism. It's not pedophilia! But these people find pleasure sexually through wearing diapers, urinating in their pants, etc. things young children would do. You want to ask him if it's sexual behaviors, or if he does it purely because it is an emotional comfort for him, not linked to sexual pleasure.
Also, you do need to see the inside of his home! If you are discussing marriage with this man, there is still alot to learn and uncover and seeing his home is a must!
Ask him to see his home, next time you are together. Finally and I would advise this to anyone, take your time in really getting to know one another and seeing if you are compatible. Then you can decide about marriage, etc. But there is no need to rush these things at all.... it's good that he shared this with you and so soon into the relationship. That means he will probably be honest about whatever you do ask him, and that he's open to you knowing him. Best wishes. Please click ACCEPT button for this answer, otherwise I'm not credited for my help today.