If he didn't eat the whole box, he will probably just have a bellyache. You may have to take him to the vet, if it constipates him. It's amazing what they will eat. Ours ate some rubberbands once, his stool was interesting for days. He passed them all with no problems though.
You might want to induce some vomiting. But if you can't do that, then you need to get him to the vet, his poor little tummy could burst from the swelling. Hope this helps, good luck.