I was kind of in this boat. Last year my fiance and I got engaged. We picked our venue and went to show her mom. We found her in her apartment. Needless to say, it wasn't a good time. The short end of it was she fell in the bathroom, crawled to her bed and didnt get back up.
It was rough for a long time. It got a little better day to day, but were getting married in 2 weeks and the PTSD of it is hitting her hard.
Anyway, the next day her dad and I went to clean up the mess so my fiance and her sister can see it. Her dad broke down so I did pretty much all of it. She kept pushing for details and I gave her a very light version for a sort of closure. Il probably never tell her what I actually saw. It's for the better.
All I can say is you'll never know what to do because you havent been in those shoes. But remember that's ok. Just let him know that you're there for him. Give him his space if he needs it, but just dont let him feel neglected. Let him go on his own if he feels he needs to. As long as he's able to without facing any legal issues. They cleared the apartment for us, and the landlord was a huge dick and said that they're throwing her stuff out in 15 days and tried to say we were responsible for damages and cleanup, but that's a different story.
Tell him he can call you or something afterwards. It's his sense of closure after the fact. Give him a little time to get a grip on things. He will probably be distant for s bit. Dont let him forget you're still there. Guys like to be the strong one.
When my grandmother passed 2 years ago, my dad was the one that held it together to set up for the family. He buried it away because he had to for the sake of everyone else. Its just starting to really soak in this last few weeks. I've been struggling with flashbacks and nightmares of the discovery and cleanup lately as well, because I didnt get the chance to grieve, because I was trying to be strong for my fiance.
When he's ready to talk about it, just listen. Sympathize. Maybe he has a friend, coworker, etc that had been in that position before. He can seek them out and they can talk about it better than you can.
That person to my fiance was her boss. She lost her mom when she was 14, my fiance was 24 when she did. It's different, sure, but it's the same pain. That helped her a lot.
Down the road, things will start to get a bit better. He'll open up a little more, be a little less standoffish and isolated. Itll come and go. It's a day to day thing.
Probably the one thing that really helped us out was the wedding. All the planning kept us busy and our minds off it and we got a lot done very early. It helped a lot with the stress and everything. Maybe theres something like that you guys can do. Not mairrage or anything but just something you can focus on together. Is there a hobby or anything you two share? Something he likes to do? Stuff like that.
I hope things work out. It sucks for everyone when that happens, but just listen to what he says he needs ir wants and let him have it.
Tldr: fiances mom died last year, I'm in the same boat as you. I'd let him go on his own. It's his closure.