I'm writing this from the wrong side of the question, and I know it.
The search people type is "why do I feel lonely married to an autistic man". I'm that man. ASD, mid-fifties, diagnosed late, good at a dinner party, bad at the hour after it. I've lived with someone for a long time. And while I was reading partner forums to write this, I had to stop a few times, because I recognised myself in the complaints.
So I'll do two things here. First, what partners themselves say, as honestly as I can report it. Then what's going on from inside the head of the guy across the table, which might help, or might at least make it less personal.
The short answer: loneliness in these relationships usually comes from connection not showing up the way you expect it to. He may not pick up on how you feel unless you say it straight. He may need a lot of time alone. He may talk for an hour about a special interest and almost never about his inner life. When he's overloaded, he withdraws. None of that means you're asking for too much. It does mean the usual signals don't work, and you'll both need new ones.
What do partners of autistic men say?
I spent time on forums like the National Autistic Society's community and AspiesCentral, where partners post about exactly this. Most of the posts I read were from wives, though not all. A few things came up again and again.
The sense of "gradually disappearing" shows up in a thread on the NAS forum and in many others like it. Partners describe slowly shrinking their own needs to keep the peace, until they're not sure what they want anymore.
Feeling like the only one holding the emotional side of the relationship. Remembering birthdays, noticing when someone's sad, keeping friendships going, explaining what happened at the party. One long AspiesCentral thread is basically that, page after page.
Guilt. That came through more than I expected. People feeling bad for being lonely, because he's "not doing it on purpose", because he has a diagnosis now, because they're supposed to be understanding.
And then, often in the same post, real love for the person. Both things at once.
What's going on from his side?
I can only speak for me. But here's what I see when I look honestly at how I am at home.
I don't always see it. If Sandra, my partner, is upset and doesn't say so out loud, there's a good chance I miss it. Not because I don't care. Because faces and tones that are obvious to most people are blurry for me. I've learned to ask. I still forget, a lot.
My battery is empty by evening. I can be "on" all day (talking, solving, being funny) and then come home and have nothing left. The person who gets the leftovers is the one I love most. That's backwards and I hate it, but it's real. I've written about why socializing wipes me out.
I show care sideways. I'll fix your bike, research your problem for four hours, make sure the woodstove is going before you wake up. Ask me to talk about how I feel about us and I'll freeze. I'm learning that the doing isn't always heard as love, and that the other person might need words too.
I disappear into things. A new project, a new obsession, and I'm gone for days. Physically home, mentally somewhere else. From my side it feels like being alive. From the other side, I imagine it feels like being left.
Fear makes me pull back. I have a long habit of leaving before I can be left. In a long relationship that doesn't look like leaving. It looks like going quiet, going cold, going into the bubble.
Has Sandra felt lonely with me? I've had to ask myself that. I don't get to answer it for her.
What can actually help?
Some things I've seen help, from my side and from what partners describe.
Saying it plainly. Not because you should have to, but because hints genuinely don't land. "I'm feeling lonely and I need you to sit with me for twenty minutes without your phone" works better than a sigh. It's unfair that this falls on you. It still works better.
Planned connection sounds unromantic, I know. A set time each week to talk, or a walk, or a thing you do together. Autistic brains often handle planned closeness far better than surprise closeness.
Couples therapy can help, but only with the right person. A therapist who doesn't understand autism may read him as cold or uncaring. One who does can help translate in both directions.
Support that's just for you. AANE runs moderated groups for partners and ex-partners of autistic people. In Québec, L'Appui can point you toward services in your region. A therapist of your own helps too. Your loneliness deserves care whether or not he changes.
And him doing his own work. He's the one who needs to learn his patterns. That's not your project. If you're the partner and this is all on your shoulders, something is off.
And one line I won't leave out: if the loneliness comes with fear, if he blows up, threatens, controls or intimidates you, that's not an autism communication problem. Read where the line is with outbursts and reach out for domestic violence support.