Reading about Maine Coons at a breeder site, the #1 trait that makes a Maine Coon is its pedegree papers. You can't tell a MC without a scorecard. (Of course, they don't explain where the first generation of Maine Coons got their papers from, there in the wild. How can a breed be both wild, but only official if it has papers???? That's daft.)
Now, onto more realistic talk. Maine Coons came from New England. It is every bit believable that Squirrel comes from the generic northeast stock of cats, and has lots of coon in him. By his coat, he is most-definitely a cold adapted cat. He has fine down hair, a bushy tail, and a thicker coat in colder weather. He may or may not be full grown.
Reading about Maine Coon personalities, and I'd say, yep, we got a coon. Or, at least, we got something out of the primal pool that produced the coon, which is far better in my concern.