Need help lightening the load...

My son is a nonverbal 3 year old who just got diagnosed in February with Autism. My husband was already a SAHD, I work full-time as a nurse and once we realized the demand of therapy I switched to straight nights.. we also have a 17 month old so I thought this would be helpful so one of us could be with our son at therapy and the other with our daughter( we don't have family or friends available to really help). Our son's sleep schedule is buckwild, falls asleep at 3 am, naps at 9pm we can't "make him stay up" to avoid the late nap either otherwise he will start to hit himself and head bang.

Well today my husband expressed that he is burned out and I completely understand... is there any advice anyone who maybe has experienced this has?