I happen to have experience with this type of situation. When I got my QH as a four year old he had been babied his whole life, blanketed, kept in a barn ect, mind you we live in FL and it rarely gets below 40 and not for long. Any way this horse of mine does not grow a winter coat, and starts shivering when it gets into the 50s. The first year I had him I tried giving him more hay, I tried just a sheet, and he lost weight and was cold, so I caved and put a blanket on him.
I read somewhere that putting blankets on them all the time damages their hair follicles or something like that. Anyway wearing a blanket prevents them from ever puffing up their coat on there own so that mechanism sort of dies. Vague I know, wish I knew where I read the article.
Back to my horse, I have had him 4 years now and even though it is still hot down here the days are getting shorter and I am noticing my others already starting with the winter coats, not my QH.
As for your guy you can always try and see how he does, but be prepared to put a blanket on him.