Yeah. I think "ice cream" is always treated like an uncountable noun in North America: "three bowls of ice cream" or "two ice cream cones" or "some ice cream." Another AE/BE difference is lettuce. We never say "a lettuce," and we consider the plural of lettuce to just be "lettuce."
"An ice cream cone" or "an ice cream bar", etc. would be correct, but if it is just some amount of ice cream, you would say "He is eating ice cream" or "He is eating some ice cream". "Ice cream" functions as an uncountable noun (if I am remembering my terminology correctly) so using "an" in front of it doesn't make sense. There isn't a concept of a single unit of ice cream.
I am originally from upstate New York but I have moved around a bit. I would probably say 'he is eating an ice cream' if there were several deserts available and one was an ice cream-based desert. Indeed, if i was going to offer to get a unit based ice cream item, i would say 'do you want an ice cream?' , but if i was to scoop ice cream from the tub, I would say vdo you want ice cream?'. Of course, that would then be followed with 'how much?'
This is a little off topic, but, as everyone us talking about the differences between English in a variety of countries, I am wondering if American English is closer to British English than Canadian English is? As I have been corrected on some Canadian spelling to American spelling (or more likely British spelling). Also, grammar is not my strength so I appologize. Also, we would have to qualify the ice cream if we use 'an/a'... with cone, bar, cake, bowl of, tub of, etc. Without a description it is a strange sentence to me. Thanks for any insight!