You can't reliably test a puppy for SM.
The only test is an MRI scan under general anaesthesia and will tell you if SM is present or absent.
Current research indicates that SM is a progressive condition and even if a cavalier is clear at a year or even two years,this may not be the case at 4 or 5 years.The likelihood of an early onset of SM in a puppy is very remote and you would be wasting your money if you were told he did not have it.
His symptoms sound like allergies or dry skin.A young puppy cannot effectively regulate oil production and can often have dry skin.This will pass as he matures.Chances are that he's still very young and acclimatising to his environment.
The vast majority of vets are utterly useless when it comes to SM.
If the nose is running clear he may have a slight allergy and if he scratches his head or ears he may have an ear infection or earmites.It's so easy to get them in long eared breeds so if you can set up a good ear cleaning regime it might help a lot.Personally,I wouldn't entertain the idea of scanning a cavalier before a year and not for such a common thing as scratching.