Optimism required!

Hi everybody. Long time listener, first time caller. Sorry in advance for the following wall of text.

TLDR: our 3yo is non verbal and limited receptive, but shows some encouraging behaviours around having a friendly, social attitude, just lacking the skills to engage. If your kid was like this, please tell me how they developed, and accentuate the positive, because I'm drowning in worst case scenarios!

Longer post:

We're going through a pretty rough patch at the moment and I'm here to seek positivity and causes for optimism.

Our 3yo is non verbal. He has developmental delay, and has poor receptive language (though has a tendency to follow instructions juuuust often enough to make me suspect it's a mix of can't and won't), plus some physical challenges (balance etc). He has consistently terrible sleep. He has pretty narrowly focused interests and simply refuses to engage in anything else that wasn't his idea - not in an openly demonstrative way, he simply ignores and redirects himself to something more interesting. He won't do pretend play (though he has once or twice in the past). When he's excited/stimulated, he bounces/jumps up and down repeatedly (in extreme moments he does it until he's out of breath). He's been in SLT and OT but his reluctance to participate meant we didn't see any progress at all.

He did start to talk about a year ago, but hit a regression and stopped. He also lost some more nuanced skills such as the tendency to walk nicely with us when out and about (now he just shoots off to wherever he wants to go). He used to eat confidently with cutlery, but now doesn't seem interested in it, being permanently distracted by something more interesting whenever we try to "re-teach" it (we've got him to eat from the fork again, but we have to stab the food onto it ourselves first). To me this feels like a sensory processing issue.

So far, so ASD, right? Well, 9 months ago we had him assessed and autism was rejected. The reason for that, I believe, is that he displays a lot of what a layman might call "non-autistic" behaviour. He's very gregarious and loves being with people (though he lacks much ability to interact with them beyond smiling broadly and climbing on them). He has great eye contact and smiles/laughs socially. He has a sense of humour. While he can get a bit funny about transitions at first, he's actually very accepting when we have to change activity. He has very few tantrums/meltdowns or any other behaviour issues. No aggression in him whatsoever, for instance (though I know that aggression is not universal in autism).

He loves seeing and being around other kids especially, but he simply doesn't know how to engage with them. His wobbliness also makes it hard for him to join in physical play, so he's just sidelined, and ends up just sitting on his own.

As I mentioned, the medical professionals (and his therapists) all said no autism. But at every childcare/daycare/preschool/toddler group he's attended, the staff "diagnose" him straight away.

My suspicion is that he does have ASD, based on his lack of speech, social skills, or understanding of wtf is going on around him. And what really concerns me is how hard it is to engage him in activities. Many sources state that good therapy is so important to outcomes, yet if he resists it, where does that leave us?

So I arrive at my request. Please give me reason to believe that he will grow and improve. We don't have "levels" where I live, so I don't know how you could grade him. I'm not expecting peer-level catch up in the next 12 months - maybe not ever. But can I hope that he will be able to feed himself when I'm in the ground? I lack any real frame of reference - and also don't even know how useful that would be - to know how "severe" a case he would be considered. For a 3yo, no speech, poor receptive language and reluctance to be taught things have alarm bells ringing for me. Am I just catastrophising??

Thank you in advance!

Edit: I just wanted to add, though I don't know if it makes any difference to anything... He is always making sounds, not babbling exactly but more like elongated vowel noises, to seemingly expressing his feelings, rather than to communicate them.