My son has Autism, He's almost 4, and we treat him with a combination of ABA Therapy and Treatments that we've learned from the
Sonrise Program. One of the things that I've found is that he will either speak when he's either in a very
good mood and very free to express himself, or when he's in a high stress situation. As an example, once I was driving with my Mother In Law, and she was trying to get him to eat a Banana (he only eats Pureed foods), so he got overly exacerbated and cried out DA, DA ,,, DA, DA, as if to say, "Save me from this crazy woman!"
Most of the time though, I think he feels that he simply doesn't have the need to speak, so he won't. Sometimes after working with him for a half an hour or so, I might get him to whisper a word or two, but usually it's hard to get him to talk. I try not to put too much pressure on him though, because I know of other parents that have pushed their kids too hard, and they ended up regressing, so I try to maintain a good balance of learning through play, but I can tell you that it is definitely exhausting work, raising a child with Autism. I've had to cut back my hours to only 3 days a week (my wife works with him on those days) and my wife had to cut back her hours to only 2 days a week, so it's been very hard both emotionally and financially on us.
I feel bad for those who end up in this situation and don't have the resources that we have.