He looks fine to me. Katie looks like a torpedo also, long and thin while her cousin Max is a large, heavily built guy. I live in NY and use a sweater/coat on Katie in the very cold weather. Max is a fluffy so he can take the cold a bit more. I use a coat on both of them when it's snowing....too much snow piles up on those long, broad backs and I don't want it carried into the house.
Seven months seems pretty young to be filling out. Ruby is about 18 months now and is just beginning to look like a fully adult dog.
He looks very much like Ruby-Doo. My guess is, he's N-O-R-M-A-L.
Living in a hot climate and having spent my adult life as a German shepherd aficionado, I'm unclear about corgi winter coats. I'd say if he seems to be unduly cold or unwilling to go outside when it gets to be colder than a bigod, then purchase or make a doggy coat him. I made one for the Late Great Greyhound...if you use the fabric called "fleece" (very cheap to purchase), you don't even have to sew a hem, because it won't fray.
Otherwise...it's never too late to move to Arizona. ;-) Ya can 't shovel heat!