No, Kami, he's just growing up. When they reach his age, they can go down to two feedings a day, so if you find that breakfast and supper are enough, with may be a snack for lunch, then do that. He'll probably eat a bit more for supper than he used to, but it's normal to switch to 2x a day as they get older.
Plus, you're right, he might be slowing down a big because of the change in weather. It's still hot up here, we're still having a heat wave, but even so, Nathaniel is in his hide where he's been for 2 days. So that could be part of it, too, but mainly they eat less during the day at his age than when he was younger. So try twice a day and see if that works for him.