Carrie I’m not sure anything I have to offer will be useful for you, ds could mimic sounds / words so I don’t really have any advice for that specifically, I’m sorry.
Ds could say words but they were just not used in context (exclusively echolalia) for a long time he did this, I used to play games with him to help his understanding, he could say loads of things but had next to no understanding (which is unusual as it’s usually they understand, then the words come) he could say over 100 words (I used to write them down) but he didn’t understand what he was saying, he couldn’t even follow the instruction ‘sit down’ he was about 4 at this point, his understanding was virtually non existent, so I had to get him and sit him down and say as I was doing it ‘sit down’ stand him up and say ‘stand up’ then sit him back down and well you get the idea, and everything was taught like that.
Then once he learnt, I used to say ds get mummy red crisps from the cupboard, then as he got better moved to 2 step, ds get mummy red crisps and daddy blue crisps from the cupboard. (Although he still is poor with 2 step instructions, he doesn’t have the attention span for it, it’s been recently put in his ehcp as a target for next year to be able to reliably follow a 2 step instruction)
I had to manually teach him how to answer yes / no with a silly game, but it worked. Once he knew the characters of tv programmes I would be really silly and say, for example using Thomas the tank, ds is this paw patrol? Noooo silly mummy, is this peppa pig? Noooo silly mummy! Is this Thomas the tank? Yesssss yes it’s Thomas the tank, clever mummy! And so forth. God that game was the bane of my life but it eventually taught him what yes / no was and then I adapted it to other stuff. He 100% answers yes / no reliably now.
One website I thought was really good and she deals with every kind of speech delay / disorder, was Laura mize teach me to talk, I remember every night when ds was 2/3 years old listening to her podcasts. She still sends me emails now which I read.