The speech therapist should have suggestions for things that can be done at home. And MadB's advice is good - when they say 'drink', you expand on that, 'want a drink? Okay'. So they know 'want' is the word they use with 'drink'.

Our speechie linked play with sentence length - so she mostly played with him - at the start he had a rubbish truck, he picked up some rubbish, that was two steps and the most he could remember, and he was using two word sentences (occasionally, lol). She got him extending the scenario; he had the truck, he picked up bins, he drove them to the other place, he dropped them off - that's four steps and four word sentences happened around the same time he could remember those play steps. I remember being fascinated! (He did make pretty fast progress, 5-6 word sentences in half a year or so - I think confidence (as usual!) was a big factor for him.) But she had us cut 'please' because it was using up a valuable word! We had a polite child who couldn't talk, lol. Now he forgets his manners but makes his point pretty elaborately. He says 'absolutely' a lot.

I'm trying to remember what else. She wanted us to put things in sight, but out of reach so he had to ask for them. We'd suggest the language he could use, we saw his wheels turn, he'd shake his head no, I'd go to the laundry and arrive back to find 2 chairs and a stool stacked and item gone. So that was a fail for us, lol. But if she sets 'homework' like trying the words 'open' or 'want' then just look for opportunities to give #1 reasons to use them.

As the therapist works with him, it'll become evident what he understands and doesn't. Time will do this too. Also - Riv's best speech therapist was probably his motormouth younger brother who FORCES him to be social.

But I think I'd play with them, just social stories, getting them to role play and become interested in more sophisticated themes and then language sort of becomes more necessary. And mostly - ask the speech therapist what you could be doing at home. If it's someone else who goes to the therapist with the boys, get the therapist to write it down?
All the best - sometimes it's a long hard slog with baby steps, and sometimes, because development happens in order, you manage to unlock something and then they're off and racing with skills!