Hmm that’s even later than my son, who had only 4 regular words at 27 months (but is now, at age 31 months talking in little sentences).
Start with signs. Loads of simple Nursery rhymes with actions. A few simple books. Counting 1 to 10. Animal sounds
What does he like?
My son likes vehicles and Peppa Pig so we focused on that - beep, beep when we saw a car, brmmm when we saw a bus, ting-ting when. We saw a bike, piggy snorts when we played with his Peppa toys.
We also learned miaow and woof woof and cheep cheep so we could name things on our walks.
Then we did words to do with movement - up, down, out.
Smile when he makes an attempt to talk, and pretend for now you understand, try to converse. Leave long gaps after questions while he finds the words in his head.
When we played I asked my ds sometimes to look at my mouth and I pointed at my lips when I made a sound, so he could see what shape my mouth made. If he was in a good mood I’d say, “you say it” and occasionally he would try. I acted really happy when he tried.
Also I’d guess from context what DS was babbling about and repeat back, really slow and simple “yes its a car. daddy’s car” or whatever. Quite often I’d guess right and he was delighted when I apparently understood.
As he got better I’d say “did you say car or cat?” And he’d repeat, trying to get clearer.
It’s a slow journey and sometimes it seems like he forgets words he knew well a week ago. But overall he’s improving all the time.