My son is autistic and he gets hiccups in his sleep they don't wake home up but last about 20 min why?

Sleep apnea?: There is nothing I've heard regarding a connection w/ autism, but hiccups during sleep have been related to the breathing pattern that can occur in sleep apnea. Hiccups are caused by involuntary movements of the diaphragm which might be set off by the disturbed breath rhythms. Perhaps his pediatrician can evaluate this and either treat him for it or refer to a sleep specialist. Best wishes.