If he is still okay 24-48 hours after the injection, I would say that he is fine. To my knowledge, most of the hedgie deaths that have occurred due to Ivermectin have involved the hedgehog showing side effects almost immediately and dying either that day or the day after. I might be wrong, but you are probably in the clear.
However, DO NOT let them do another injection on him!!
Next time you go in, insist on having him treated with Revolution instead. It works just as well, if not better than the Ivermectin, and no one has reported any serious side effects from it. Your vet will almost definitely press for the Ivermectin but you need to stand your ground. Even though Remy didn't have a bad reaction from the first injection, it is still possible (and very easy) for them to overdose him with the next injection. The risk is not at all worth it.
If your vet won't do Revolution and keeps trying to get you to do Ivermectin, I personally would walk right out of there. Many vets, though, are understanding and will do what makes you comfortable. Perhaps when you go in you could print out some information about the Ivermectin causing death in hedgehogs and take it with you. I did that with my vet, and she actually started recommending Revolution instead of Ivermectin to the other hedgehogs that she sees!
You might save more lives than you know!
