They really all vary. Most cavaliers I have seen, and all of mine do not have long hair on their back. This includes my dogs from a show line and my rescue dogs. Their back hair varies from about an inch to two inches long. Cavaliers aren't like, say, those show terriers just in Crufts , with really long back hair that grows to the ground!
Coats really vary! The breed is supposed to have a straightish coat (though chests are curly) and show breeders as noted will dry the coat out straight for showing. Mine all have fairly straight coats but Lily's (rescue) is dry and more curly and needs to be kept trimmed to look neat. Otherwise she looks like she stuck her paw in an electric outlet!! :lol:
If a dog has no longer hair at all on its side, and is a mature adult, that is a bit odder. If your dog's from a reputable show breeder, then I would certainly ask them about it -- any good breeder will want to know about a coat anomaly, or it may be that this length is right for this line. (people perceive things so differently that it is hard to know without seeing a dog whether things look totally normal or a bit unusual!). Great breeders keep lifelong relationships with their puppy buyers and will be happy to answer questions.
If the coat is really abnormally short, then I'd wonder if the dog has been repeatedly shaved -- this is never recommended for cavalier coats and some breeders say it can permanently damage how they grow and look; or whether there's a chance that there's a different breed mixed in? Maybe not in the immediate past but somewhere further back, as from your little pic, Charlie looks like a full cavalier.
If he's of unknown background (for example, a rescue dog) then it is hard to know for sure about reasons for short coat length.