How long do I let him sleep? (Toddler with tummy trouble)

Mandi 😈💀 • Expecting #2 August '15!
My 20 month old son has had some tummy trouble the past few days, blown out several diapers and required a few emergency showers to clean off sour smelling diarrhea. (Sorry!) We fed him a BRAT diet yesterday, and he seemed to be a little better, the frequency of poops went down. The woke up at about 6:30 this morning with a mostly clear almost water with tiny flecks of poop, which when he had this one time before was when the was done with it. I brought him to my bed, he farted around and played for an hour or so, then fell back to sleep and is STILL SLEEPING. It's 12:15 here now, and I'm getting a little worried because normally he would have been up, had breakfast and a snack and water and milk by now and be getting ready for his nap. I don't want to wake him up necessarily, but I'm not sure how log I should let him sleep when he hasn't eaten. What do you think moms, better to let him sleep until he wakes up, or try to gently wake him up for a little snack and go back to sleep if he wants? (And for those who will ask yes I've been in contact with his pedi about the diarrhea and he has no fever or other symptoms except the sleeping now)
 Thanks all!