In the nicest way possible you are way overthinking this and I say that as a parent of three autistic boys aged 4 to 13. I get it, I've obsessed over signs and symptoms too, but it doesn't help anyone, least of all you.
Unfortunately the support system for these things is awful to non existent in many areas and I would highly recommend focusing on your sons needs as an individual person rather than as a label. He can't be Autistic Spectrum Disorder. No one can be ASD.
My boys' father also pursued a diagnosis as an adult, with a strong history of autism related diagnoses down the male line. He bitterly regrets it. Of course that might not be the same for your husband but on this side he was looking for some degree of understanding about himself and his behaviours that didn't come with the diagnosis, not least because he quickly realised that all autistic people are wildly different from each other anyway.
The spectrum doesn't run from 'mild' to 'severe' - it just doesn't work that way. It's more of a 'web'. There is no effective way to assess a baby on these traits, nor is there any way to predict 'how autistic' someone might be, that's something that has a flow of change, much like the growth and development of any other human.
To use an example the diagnosis report of my eldest son was completed when he was 6 years old, almost none of the information in that report applies today. Is he still autistic? Of course. In the same ways? Not at all. The report and diagnosis itself offers absolutely nothing useful to the support he needs today.
When my now 6 year old was 2 he was diagnosed autistic alongside a chromosomal duplication, at the time you'd think he was a genius, taught himself to read, times tables up to 15, non-verbal and communicated only via magnetic letters on a whiteboard. Now he could talk for Scotland and spends the majority of his time playing daft games and avoiding homework like most other kids his age. Most of his 'genius' got lost along the way, he barely remembers any of the things he'd once repeat 800 times a day. He's still autistic, he still struggles, just in completely different ways than he did four years ago.
My now 4 year old has the same diagnosis and same duplication. He is what you might call 'severe' in that he is barely verbal (on par with an 18 month old at best), not toilet trained, sleeps a maximum of 5hrs a night (broken) and has for as long as I can remember. He is attached at the hip and needs constant focused attention. I would be naive to think he won't be like that forever, but equally I'd be a bit daft to assume he'll always be that way, because he's only 4.
The labels of ASD/autism/aspergers/HFA/severe autism don't actually help anything. They offer a vague understanding of some struggles that some people might face, which frankly is not much different from saying someone has an unnamed physical disability. Without the information of the individual person and their experiences, a label of ASD offers no more insight than any other you could pick out of thin air. Each experience of autism is as unique as the person themself.