Hello from Canada!
Does he have access to hay at all times? is he of good weight now? does he have good shelter that will block the wind from him?
15 degrees isn't cold for horses - if you are doing a full body clip, a good, waterproof rainsheet should be fine. (assuming he is in good weight now and has lots of hay to eat).
We have horses with trace clips (about half the horse is clipped) and they go out naked if it's 15 and sunny. They have access to hay outside at all times (eating keeps them warm).
We always blanket according to the windchill, not the actual temp - so, if it's a calm, sunny 15 out - everyone is naked. If it's 15 with rain and a good wind (therefore the windchill would feel under 15), then we put rainsheets on them.
When blanketing, horses should not feel warm under their blanket. It's healthiest for horses to feel just a bit cool (you know that temperature that you wouldn't mind having a long sleeve shirt on instead of a t-shirt because you are just a little cool - but, you can totally survive in the t-shirt? -that temp is what horses should have).
The wind is what makes them cold so, if he doesn't have shelter outside, that would be a factor in putting a blanket on him or not as the blanket has to serve as his shelter.
There is a really cool app - I think it's from one of the blanket companies - you type in the temp (factoring in the wind) and how your horse is clipped and it tells you what the animal should wear. Obviously not perfect but, it's pretty accurate.