The first step here is to decide: What is my ultimate goal? In other words, what is the most important outcome for you in this situation? What are your must haves? Is there a deal-breaker?
Without knowing any more about the 30-yr old son, this conversation (and by the way, there must be a conversation!) could end in a number of possible ways: You continue to "date" your friend but not move in together; you and your friend live together with his 30-yr old son; you "break-up" with him because you decide if he won't take the relationship to the next step, you're not interested in being with him at all; or, your preference--you and your friend will live together without his son.
Only you know what you're willing to do.
At present, your connection is convenient. You have an exclusive relationship in which you both enjoy each other, yet there are no plans to move in together. If you were in your 20's or 30's, and you wanted to further your commitment and you wanted to have children but he didn't, I'd encourage you to leave him. However, since that isn't the case, I wonder if it still would be OK with you, in the end, if he wanted to leave things the way they are.
I don't know how impaired his son may be and in what ways. I'm thinking there probably is a way for the son to be living away from his dad, because you didn't mention anything specific to the contrary. I'm wondering then if your friend's son is a convenient impediment to his moving forward with his own life. You mentioned he is a widower. I'm not sure when his wife died in relation to when you and he started seeing each other. As I'm sure you know, it takes each person a different amount of time to accept a spouse's death. Your friend has been in a serious relationship with you for four years. However, he may not be ready to picture himself living with another woman. (Has he slept over at your home and vice verse?)
I am most struck by your last comment, "He's not much into talking about what to do about the boy." There may be many reasons for this but we really won't know why until we ask. I'm thinking you are the one who needs to begin this difficult conversation. Begin by telling him how you feel about him and how much you enjoy being together. You can ask how he feels about you. I'd be surprised if his feelings were not mutual.
You may have even talked about "the state of your relationship" before. If you haven't already, explain to him why you want to live together. This time you need to find out how he feels about living together with you and what,(if anything), this subject has to do with his son. Hopefully you will each speak truthfully about your relationship and vision for the future.
I look forward to hearing about your "heart-to-heart". My guess is, there may be more than one outcome that would satisfy you.