It sounds like a slow crop. Check things like the temperature of the formula, was it too cold? That can cause slow emptying. You also don't want it too hot either, a water bath with water at the appropriate temperature monitored with a thermometer is extremely important, as well as mixing it thoroughly throughout feeding to maintain a consistent temperature. Is he warm enough? They have trouble maintaining their body temperature and using a heating pad on low under half the bin he's in can help him regulate better, moving to either side when he's too warm or too cold. Thickness of the formula is important too, is it too thick with not enough water? He could be dehydrated causing a slow emptying crop as well.
There's a ton of variables at play here. Check the formula for temperature, and possibly make it a slight bit thinner during the next feeding, and make sure he's warm enough. Make sure he's pooping as well and check the consistency. If his crop is being overfilled and stretched you might also need to make a "crop bra" to help keep it in alignment. If it doesn't resolve soon, or happens again, he needs to go to the vet. He can very quickly become ill, with a sour crop that might need to be emptied by the vet.