How does one measure progress with special needs children? Move the goalposts.
In other words, wherever you are, revise your goals so the current state represents progress. It's better to choose a new measure rather than simply shrinking an old one, but heck, you do what you need to do.
In the real world it often is true, though illogical, that real progress follows the fraudulent sort. Corporations do this all the time; a product that's 18 months late to market is always redefined as a great success. This becomes true more often than you'd think.
So it is with children. Be creative. Find a metric that allows you to declare victory. Then celebrate. Then find another one ...