Okay,
For us, between Wake up and snack time can be, snuggles (or something) feeding the dog...then breakfast, playtime and maybe a few cartoons.
They "help" with household duties...
For your DS, is he able to have independent "A" time? Does he have a chair or play centre where he's able to keep himself occupied? How is his upper body mobility? Is he able to reach for toys? or grasp objects? How are his verbal skills?
Being that his wake up time varies, and you are fairly new to this...What about approaching his day with an EASY attitude, but not necessarily focusing on times? The idea of an EASY is to operate on a 12 hour spectrum...so, from wake up time, 12 hours from that would be bedtime. The theory also supports earlier bedtimes and the fact that it promotes a longer sleep time. (DEF. works for my kids). My sis's kids can go to bed at 10pm and wake up at 10am, doesn't matter...my kids wake up same time give or take a half hour.
so (you insert times)....If possible set his dr's appts and therapy around his "A" times (although I know that's not always possible)....
Wake
(E) Breakfast
(A) playtime
(E) snack
(A) craft/story
(E) Lunch
(S) Nap
(E) snack
(A) playtime/walk/park
(E) Dinner
(A) playtime/bedtime routine
(S) Bed.
Ideally a set bedtime, set eating and nap time, and learning your child's cues will let you follow the routine to your families specifications. And he will begin to associate certain things with other things. My kids know that shortly after lunch that nap/quiet time will follow. They have also learned to predict when things will happen. They know that once bath time happens, the wind down for bedtime is on the way. Just remember, this is a predictable routine, not a strict schedule that you must follow at all costs!
Take it slow....and good luck!