Demodectic mange is a very common problem. With a quick search of these forums, you would quickly find several hundred threads on the topic (actually, there are thousands).
The problem behind demodex is actually immune system function (or lack of it). Every dog on the planet has demodectic mites, ALL of the time. You have them too, btw - (a human version), which live inside your hair follicules. And if it happened that you had an immune system problem, you might likewise suffer from bald spots
The key to treating demodex is thus support of the immune system function. In the case of a puppy, the problem may simply be that his immune system hasn't yet developed sufficiently to cope with all the stresses thrown at it (things like rehoming (major stress), vaccinations, flea, tick and worming are all enormous assaults on the immune system, and puppies get it all in bucketloads. Sometimes, their immune systems simply aren't able to cope with all that - and then you get problems like mange (when they're not even able to keep a harmonious balance with their mites, which are a natural and permanent flora of the coat/hair).
In these cases - and it's a good 90% of them, the only treatment required is support of the developing immune system. That is, good quality diet (for which most commercial kibbles don't qualify) and suspension of any further stresses until the mange is under control. That means no vaccinations, flea/tick/worming treatments only if absolutely essential, and no elective surgeries such as spay/neuter - until the pup's immune system is at least strong enough to deal with everyday life (i.e. the mange is gone).
In a few cases, about 10% of mange sufferers, the mange develops beyond a few spots to what is called generalised mange. In this case (or with adult dogs who develop mange), it's unlikely that you're just dealing with an underdeveloped immune system, but a faulty one. Even amongst these individuals, 40% or so will still recover without any treatment. It's really only about 5-6% of all mange sufferers with such weak/defective immune systems as to be incapable of recovery on their own, and in need of aggressive treatment.
If your puppy has only a single spot, he's certainly not in the generalised category (yet, anyway - can't guarantee the future) and should be treated with support to the immune system. Do bear in mind that mange will almost always become a little worse before it gets better (there's a delay between damage to hair follicules and the hair actually falling out).