ELI5: Why does my autistic son keep getting tired?

My son is 5 and has been diagnosed as level 2 autistic, but lately, his energy levels have been really uneven. In the mornings and evenings he has plenty of energy but in the middle of the day he’s not energetic and just sleeps a lot. We make sure he gets a good night’s sleep and his doctor hasn’t found anything that’s wrong with him. To top that off, he doesn’t like being outdoors, so that doesn’t help. Why does he always get tired like that during the day?