Soumia it would be useful to know what level of receptive language he already has? What sort of instructions does he currently understand?
Receptive language is what is most important in my own opinion.
I've been in the situation when ds was younger were he could 'speak' but didn't understand a thing I said, which isn't too uncommon with autism. (his speech was purely echolalia) he would recite a full sentence but didn't understand one single word he just said so was pretty useless having the speech at that point if you see what I mean as he couldn't do anything with it because he didn't know what he was saying. So the speech wasn't functional.
He didn't follow instructions, didn't understand a basic command like 'sit down' this was age 3 to 3 and a half. it makes things much more difficult if they don't have receptive language.
What we did which took forever to get going but he slowly got the idea was sit him down and say 'sit down' stand him up and say 'stand up' repeat repeat repeat. Then when we thought he understood that, as in, he would sit down unaided when I said sit down or stand up when I said stand up we would move on to a couple of other instructions to teach. Daft things like where is mummy's nose? I would have to take his hand and touch my nose whilst saying it what seemed like a hundred times before he could do it unaided. And so on and so on. We eventually made it harder when he got good at it so instead of pass me the cup it would be pass me the blue cup (once we had taught him colours) Etc.
He did not understand how to answer yes or no questions even age 5 and it took me many months to teach this. I played a silly game to start with. (he often wears character pyjamas) so once he knew some characters, I would say for example if he had paw patrol pyjamas on in a really exaggerated voice 'is this Thomas the tank? Noooooo silly mummy! Is this peppa pig? Nooooo silly mummy! Is this paw patrol? Yessssssss clever mummy! Emphasising on the yes and no bit. I swear it was that game after playing it a good few times that taught him how to answer basic yes/no questions, then we worked on other yes / no questions.
Point absolutely everything out to him, label absolutely everything, name every single thing you are doing with him. Every action.
Ds is walking up the stairs, ds is walking down the stairs, ds is throwing the ball, ds is catching the ball. Ds is drinking his drink, eating his dinner, Basically anything and everything to make him understand.
I use to and often still do feel like I'm speaking to a brick wall but it's being absorbed even if I don't think so.
Sorry this all maybe irrelevant, he may have this level of understanding already. It's not clear from your post.
Anyhow, you sound like your doing everything and more already. Don't burn yourself out. We never did aba so can't personally comment on that, you sound like you have your mind set on aba?
My ds is almost 6 now and whilst his receptive language isn't anything near age appropriate, he understands much more than I ever thought he would at this point and shows signs of improvement all the time.
Teach me to talk has a good website I used this a lot in the earlier days, lots of tips on receptive language.
Good luck.