Horses can withstand freezing temperatures as long as they have their winter coat and have enough food to eat. Food fuels his digestive system which acts as his own personal heater. If his coat is only just growing back then I'd pop on a lightweight, but if he's really fluffy then I wouldn't bother.
My mare is wearing her lightweight stable rug tonight - it's -2°C where we are! She doesn't really need it, but since she cannot generate heat through movement as she's currently stabled at night I've put it on to ensure she doesn't get chilly. Though if it's 1°C I don't bother - she's got her thick winter coat and enough hay to keep her warm.
It depends on the temp outside but I normally put on a cotton, then a fleece or woollen, and then a rain sheet if their coat is coming in, but I suggest a hunter clip, is great if your not wanting to constantly worry if your pony is cold!
He lives inside, and doesn’t really sweat that much right now. It’s generally about 10° warmer inside our barn...