Well, if you've learned anything useful about those of us who are afflicted, if we don't want to learn something, we simply will not. Make sure this is something that he wants to do, or you will be wasting both yours and his time.
Use a spread sheet. Have a checkbox for each day between lessons. Make sure each item s/he is to practice is concrete, including the number of repetitions. The more specific you make it, the higher the compliance.
If the kid plays video games, it`s too late.
I have a high functioning autistic son...and if this boy is not interested in learning the piano , he won't. These kids don't fit into common molds. If however, he wants to learn to play , you may have a savant on your hands. If however he is interested in the history of things then teaching him the origins of the piano and why it would be good to learn might be helpful. Although this may be the parents' job.
plain routine every time he comes and baisicly... autistic children decide them self what they want to lern... (they know were their talant's are)
start with a routine he enjoys (let him help decide the routine). If he is unfocused, try holding his hand while you verbally redirect him. I have found these children are comforted by touch. Allow him some choice in what he learns. These kids will often surprise you in their abilities when they are allow to choose the activity. Often regalia (concrete ob<x>jects) are your best resource to use to engage them. These children are usually focused on one thing and cannot move on until the thing they are focused on is resolved or fixed. Hope this helps.