Sounds like your guy is an easy-doer. Don't you worry about him! My gelding is one of those, too, and he is 24/7 pastured along with 11 of his herdmates, among whom there are both heavy-built drafts and fine-boned warmbloods, and some trotters as well. All of them receive free-choice and good quality hay, salt licks and also oats once a day, and none of them require rugging. I wouldn't hurry with blanketing your guy, too - just observe him and watch, how is he doing, and, if he seems warm, just let him care for himself. I'm sure he'll do just fine even in the lowest temperatures - our horses are warm and comfy even when the temperatures drop below -30 by Celsium.
I have come a long way, to surrender my shadow to the shadow of my horse.