Hi and sorry for the delay--I think my last reply got eaten.
My son was (and is) definitely delayed in expressive communication. But it was always very clear that he could understand things, and complex things. His expressive language still lags behind that of his peers, but he has caught up several years in the past 12 months. ASD kids will have uneven development. But unless he has an intellectual impairment, don't treat your son as though he doesn't comprehend language. He probably is communicating (sounds, pointing, cries, etc.) but you're not attuned to him. You're expecting him to communicate in the way that you do, and his brain isn't really wired for doing that easily.
With my son, I kept treating him like a child who understood all of what I said, even if he didn't show the responses you would expect of a typical child. And although he has an unusual accent and intonation, his speech is coming along really well. He integrates concepts imaginatively, can respond to direct questions, and has a great sense of humour.
So, what can you do with your son, especially as no one is around? Are you anywhere where the NDIS is , or is coming soon? If so, you should be able to access speech therapy, even under the telehealth (e.g. video appointments). I assume there is a local school? There must be an ECDP (Early Childhood Development Programme). Even if your son isn't diagnosed, they accept kids from 2.5 years and do speech and functional skills there.
As for things that you and your husband can do, this is difficult because I assume neither of you are trained in speech or behavioural therapy. In case no one has mentioned it: you will not love the autism out of your son, so start thinking about the practicalities. What can make a difference is for you to become trained in one or more areas of (evidence based!) therapy. Avoid anything expensive; it's usually a con. Look into DIR floortime; a lot of free stuff on the web will show you the principles. The evidence for it is not great, but it's a good framework for child-led interaction and trying to meet them halfway, instead of expecting a child to communicate in a manner that is suitable to you. The Hanen 'More Than Words' system is something that speech therapists recommend, but I'm yet to see any evidence that supports it. And it's absurdly expensive. But promises to be life changing. Beware of those combinations.
Something with actual evidence behind it, and will help develop language (even if that isn't expressive language) is the PECS system. I attended a PECS training course (two days) and we ended up using the system (to augment language; not as a complete substitute) and it cut down on my son's frustration when he couldn't easily identify what he wanted because the thing he wanted was out of sight, or a concept, or an experience or whatever. There's quite a bit of time investment (from you to learn, and then to teach him how to do it), but it's ultimately a very cheap system that will help cut down frustration and give him the architecture for sentence structure.
HOWEVER, if you live somewhere where there isn't a school with any special education teachers to lead the EDCP, or any speech therapists, or developmental psychologists, etc. you should probably consider moving. Whatever reasons you might have for staying, are they as practical or important as giving your child appropriate medical care so that he can develop the skills to participate in the rest of his life? Early intervention is the best chance he has, and this has become a whole lot easier since the NDIS is here (or nearly here, depending on where you are).
Feel free to ask if you have any questions about specific therapies. I'm a social scientist by training, so I can usually find out if something is evidence-based. (Although I think this sites therapy directory is really good at that.) It's a really difficult time and you're going to want to have a lot of social and professional support around you. Best of luck.