DS is similar (although he's 26 months). The advise I've been given by the HV is lots of choice questions - so rather than 'do you want juice?' it's 'water or juice?' Also, we have to try and get him to engage with books - we've always read to him and encouraged him to point at the ball, the dog etc, but now we need to move on to 'what noise does the dog make?' And so on.
We are having some success with the questions, once we found the right ones. 'Water or milk' gets nothing, because he doesn't mind. But 'Would you like a banana, or would you like to get down [from the table]?' gets us an attempt at 'nana', because he really does want the fruit! Also, on walks we let him out of the pushchair whenever he says 'down' (this is in the woods so it's safe), rather than allowing him to tell us with his body language. Basically, we're playing a bit dumb with him.
I am also making a real effort to simplify my speech a bit and emphasise some words (so BANANA and DOWN in the question above, for example), so he has the best chance of making the association between the word and the thing.
It's too soon to say if this will crack it, but I think we're making very gradual progress. He's always been a child who needs to have a reason to do new things - he needed to be put in places where crawling wasn't practical (outside on the grass, he didn't like the feel on his hands) before he would walk and so on. Once he's seen the point of something, he's off, but I think he's a bit of a minimum effort for maximum output sort of person.
The SALT team here does drop-in sessions every so often - we're hoping to get to own tomorrow, as it happens, so might be worth asking your HV if anything similar happens where you are?