Some suggestions for your 6 year old to have success are having visual directions, sequences for him. The best way for him to handle understanding the different workshops would be a picture schedule that is used at church and at home. I would ask the parents what they would suggest for him to be successful.
As far as the computer part goes....that is strange that they would not want him to do that one, it will be harder on him to NOT to do that one when his class is there doing it. Special needs kids tend to really enjoy the computer. And they get used to routine.
Kids with autism have trouble understanding or adjusting to change.to If schedule change in Rotation happens without notice to them, they get stressed trying to figure out what is expected of them next. I don't see rotation stressing special needs kids because it has a schedule and each workshop has a concept and theme they can look forward to. With notice from parents and your schedule posted it would be great for him. Parents might have the workshop schedule visually displayed at home and explain it each week. Advance tours might help too.
Churches will be seeing more and more kids with autism and we really need to be ready for that.