Hello, I'm Dr. Deb and will do my best to help you today.
I'm so sorry that Sebastian decided to eat a ribbon but, unfortunately, this is all too common a problem with cats:(
If this were a dog, I'd advise trying to get him to vomit the ribbon back up; however, this wouldn't be the wisest course of action with a cat since there's a relatively high risk of aspiration pneumonia developing after trying to give them hydrogen peroxide. It's just not worth running the chance of this serious condition developing by trying to make him vomit at home.
Therefore, the following options are available to you:
1. If he ate the ribbon within an hour or so ago, you could take him to an ER hospital so that they could induce vomiting with safer drugs.
2. Take the wait and watch approach which means that you'll feed him now and monitor for any signs of an obstruction....which might take anywhere from 24 hours or longer to be seen, depending on how long the ribbon might remain in his stomach before moving into his intestines.
I'd also examine his stools every day for evidence of the ribbon.
Cats can pass these sorts of objects but they can cause an obstruction as well; there's just no way of knowing what will happen. Signs which might indicate the latter problem would be vomiting, loss of appetite, lethargy or abdominal pain. If any of these signs should develop, then he should be seen as soon as possible.
I hope this helps. Deb