That would depend on how good of a winter coat he's grown and how much hay he's being fed.
it depends on the weather,season, and the kind of blanket
Winter I should imagine.
winter
A horse should have a blanket in fall or winter to keep it warm.
There are two answers to your questions:If you are asking "if you need a blanket under a saddle" then, yes. It is to protect a horses back.But, if you are asking "if you need a full body blanket to keep a horse warm" then, no. Unless you are in Norway and you have a very thin coated horse.
If you blanket a horse at the beginning of Autumn then it has to stay on until warmer weather comes around again as the blanket prevents the horse from growing it's winter coat. Without the the natural hair coat or the blanket the horse could become susceptible to influenza. You can also blanket a horse with a thin blanket in the summer. This keeps the flies away and prevents the horses coat bleaching in the sun.
winter and fall but mostly winter
Winter
Usually you'd get your horse a new blanket in late fall or early winter. Or what ever the coldest season for your region is.
It all depends on what the weather is like, eg. one summer i had to put a winter rug on it. Also, if the horse is clipped it will need a rug on all the time besides when exercising it. I would say Winter and Autumn you will need a thick rug on including in the stable and spring and possibly summer a light rug maybe in the stable, DEPENDING ON THE WEATHER!!!
The answer is pretty simple... WINTER!Winter
water blanket and food so he/or she can stay warm and be not starving