I was advised to do sing and sign. There are a few makaton sessions on YouTube. Nursery rhymes help with actions like row your boat. Maybe getting objects and putting in a pillowcase and making it a game. My daughter had actions for stuff she wanted to do like she would mimic mixing for cooking cakes or jumping for trampolining. I was told to take photos of things beginning with b so bear, ball.
Search 'communication board' there's a lot of variations of printables that might have some high frequency words to start him out asking for things. Also to add to what was said above, signing really helped my boys start talking too
Now that he's diagnosed if you're in Australia start looking for a speech therapist she will guide you on ways to help your son but she will also help your son speak. My son was diagnosed the age of two with level three autism. He's been non-verbal until six months ago he's five years old. He's only started to speak words as of four months ago so he's quite late and Speech he was exactly like your son, because we were stuck in lockdown with Covid he never had contact with Kids for the first two years of his life which delayed his speech even more we once we come out of lockdown and be with other people I started him on speech therapy with Pinarc And Speech has come to fold. I also worked with him at home, with flashcards you do need professional help to Guide as well. I'd be looking into a disability service that has a speech therapist and connect with them and start Therapyas soon as possible. It helped my son but it took a lot of work and therapy
A pecs book would be a great start! My son is in delayed in his speech and he communicates so much with his pecs book! We absolutely love it.
I made ones for my son by taking photos of things he knows around the house to help him communicate as he is aware of those items. I will be doing another set with the words under. He turned 3 in September
https://www.instagram.com/reel/DBduxcIMVO0/?igsh=MWRtaHozZHBwMWN0bA==
This video uses those exact puzzles!!!!!!
Pecs and makaton would be a good start