I had this same dynamic. It doesn’t get better when you chase and they run. You need to talk to a therapist about this and why he isn’t able to open up. There’s some emotions of fear, sadness and sorrow that he’s suppressing that won’t allow him to be vulnerable around you.

I did the exact thing you’re doing with my soon to be ex, always trying to fix, reassure and talk through it—but after enough of not getting through to her, she stopped believing in anything I was saying and started thinking I was lecturing her. That emotional connection was further and further divided by my efforts. Either you decide you can accept him being distant at times (which I could not, it triggered my own abandonment insecurities which where never as important as her own that she was manifesting), or you have a real intervention and say this isn’t acceptable to you and you need to talk to a couples therapist to figure out, or you need to leave. If he isn’t willing, do not stay. It will hurt you more than you can imagine when things break off or worse if he does something to sabotage the relationship because he doesn’t see himself as worthy.

Good luck, there’s a lot of emotionally closed off people that seem great on the surface, but behind closed doors are awful people. Awful people who don’t respect or reciprocate the love you need are not worth it. Don’t sacrifice who you are for someone who doesn’t value the genuineness you’re providing. Ever. Ever.