If he has a good heavy coat of his own, he probably doesn’t often need a blanket. If the weather is seriously foul, he should have one that is waterproof and breathable. There are many options. The best thing you could do is to build a run in shed so he can get out of the wet and wind. If your dad can do it himself, it isn’t really expensive. Generally, horses do better when they can live as natural a life as possible.
It is never the horse's fault