If there wasn't much ice cream and it didn't contain a large amount of real chocolate (artificially flavored?) or dark chocolate (as opposed to milk chocolate), you shouldn't need to worry about the chocolate toxicity factor; however, the ice cream itself, could cause gastrointestinal upset for your kitty, as most cats are lactose intolerant. His pancreas could also be adversely affected if it was high in fat, but if he didn't ingest a large amount, this shouldn't be a factorhttp://www.cat-world.com.au/chocolate-poisoning-in-cats
Watch for rapid and/or labored breathing, hyperactivity, rapid heart rate, tremors, seizures, vomiting, diarrhea, and general symptoms of illness like decreased appetite and/or lethargy. If your cat begins to exhibit any serious symptoms, have him evaluated in person by a vet, immediately, for supportive care.
I hope all will be well with your furry boy!
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