Can I just qualify my post. DS's attention span was nothing like that. It quite possibly is nothing like that in a busy classroom, although I think it is within the realms of normal in that he will sit and listen in carpet time, just as long as his peers, even if he doesn't understand everything.
As I'm sure you must know, anyones attention span is longer when the content of what you are attending is interesting and motivating for you.
A 3 hour ABA session will be very carefully structured so that the pace is fast, there are breaks that require attention but less demanding stuff, but the programme is indivualised and his tutor is 'tuned in' to his level of attention. When he looks like he's losing interest the game is hyped up, when he is highly alert and attending, the task might be extended to get the most from him.
However, he has been doing this for so long now that he understands HOW to attend, even when it is boring, so his attention span is quite long, probably longer than most of his NT peers. I expect he has been 'trained' into it.
But I think the important thing to remember is that attention span is not innate or a consequence of a disability. It can affect it, and in cases such as ADHD possible quite significantly. But short attention span is not part of the diagnostic criteria for ASD.
Now, if the child is not understanding, language they are hearing is too complicated, they're confused, bored, pace too slow etc etc. then the attention span is likely to be less.
I would suggest that you ask the teachers what strategies they have or are going to put in place to extend your ds' attention span and keep him focussed for longer and how they are going to measure the impact of this.
They could start with a very rewarding activity and measuring how long he can attend that, then try again every day to see if it can be extended, using both your ds' improvement and the improvement of their own ability to engage him. This can be transfered over to a less rewarding situation.