Okay, assuming he's acting normally and not getting cold you'll probably be okay untill it gets light out, but it's hard to tell not knowing how long it's been since his last feeding. How's he acting? Sleeping a lot? Mewing? Does he feel cold or warm?
Sorry for all the questions. I've fostered a ton of kittens that age and I know how quickly they can decline. Just want to make sure all the bases are covered.
At the risk of sounding like a mother hen, you really need to get kitten formula for him first thing this morning. Like seriously first thing. At the butt-crack of dawn.
ETA: If he probably just opened his eyes, he can't be more than two weeks old or so. That's way too young for solid foods, they don't start eating mush or dry food untill around one month. Feeding him mush may not be the best thing.