Unless he is clipped or it is below 10°, he doesn't need to be blanketed. Of course, there are always exceptions. I had a horse once that just couldn't handle the cold. He shivered so badly the first winter I had him that I invested in a blanket.
My horse is fly sheeted all summer to keep his colour and dapples from fading then he is moved to a sheet in fall and now is in a winter blanket. He still grows a big coat but blanketing keeps it manageable, saves him from the wind and after I ride if he's a tad bit sweaty he's ok to go out. Also it keeps him nice and clean and I'm a bit of a OCD about that lol. I live up in Canada and the winters get very cold and with the wind can get to -45C or worse so I think they appreciate it that they can be nice and cozy and not have to hide in the shelter all day. If I didn't show or ride my horse he'd only be blanketed on the cold days.
I live in an area where winter can be rough, but I rarely blanket my horse. If your horse doesnt have a shelter I would definitely blanket. Or if your horse is clipped you need to blanket. Blanketing can be kind of a hassle though, and you have to make sure you don't blanket too early so that your horses winter hair can still grow in all of the way.
I prefer to blanket because our horses usually continue to work quite intensely throughout the winter and we live in a cold climate so we blanket so that they don't grow huge coats so that they don't take hours for their sweat to dry after riding. We also have to clip many of our horses for the same reason and therefore blanket to keep them warm.
I'm in Australia, I think it really depends on the horse, what they're used to and what you do with them.
Some of my horses only get rugged when it's very cold and raining, others are rugged all the time as they're more sensitive to temperature/weather/bugs/general environment.
My main horses are rugged overnight to keep the chill off and then nude through the day unless it's raining.
I usually never blanket, but with the arctic winds that came through the US recently, I wanted my three horses to have some extra protection. I originally purchased the blankets for cold rainy days. They're waterproof 1200 denier, no fill. I want my horses to have fluffy coats, which they do, but that fluffy coat is useless if it's drenched! I also may or may not be pinching pennies for hay costs :^)c Less cold=less hay piled up for them.
I also have one mare with a thinner coat, who was also heavily blanketed all her life, so I can rest easier knowing that she especially has some protection from wet conditions.
I don't blanket I'm just curious what everyone else is doing and other people's opinions.
If ur blanket is to big or small it can leave rub marks on the chest/shoulders
I prefer to blanket I have 2 indoor horses I have stall sheets on them in the winter and I either take the off and put winter one on in the morning or I double blanket depending on how cold it is I also have rain sheets for those chilly or rainy days and fly sheets for the summer my 2 don't mind being blanket just make sure the blanket fits