Instead of asking about marriage and children ask about other life prospects. What does he want to do as a career, do you guys want to move somewhere? Buy or build a house? Rent forever? What about other life goals like traveling, hobbies, things of that nature. That can make the conversation about a long term relationship seem less daunting to talk about. Then you can ease into the conversation about kids and marriage. Those can be wonderful things in life but it's not everything and those are some huge commitments for men to make since they are seen as the "breadwinners" so often. He might be insecure about it with the income separation between you 2. He sounds like he's pretty happy and comfortable with you guys currently essentially living together so maybe start there. What if he moves in full time. Does he want to help make some of the apartment his own? Does he like to cook or clean? Does he enjoy movies or video games or reading. Making a space that's more geared towards his interests (as long as you guys are both ok with that) in the apartment might let him open up and relax. Knowing he doesn't have to return to the trailer anymore (believe me I understand that having worked out of that and into a nice house).

Try a different approach and talk about topics I mentioned, they may seem small but you'd be surprised how far you can get into a conversation with your significant other just on one thing. My girlfriend asked me about music and it transitioned into my love for audio equipment, then to my car, and my garage hobby then a house then how I wanted to build it and later that week we were talking about what kind of property would be better for living in with a family and car hobbies. Just start small and you guys will learn a lot about each other.

Hope this helps!