It is a good time of year to buy snow gear in the sales. I buy the padded waterproof trousers that you would wear for skiing in, ski jackets, waterproof gloves, fluffy soft thermals padded fur lined winter boots, hats with ear flaps, earmuffs and buffs for mine. If you have a mountain life near you or an H&M then I can recommend their snow wear.
I make sure the trousers are a neutral colour that can be passed on from dd to ds and then sell them or pass them on to friends when he has outgrown them. They wear their jackets as their school/everyday jackets so they get well used and again, the gloves generally get handed down.
I don't hand the boots down but they do sell on ebay. Dd has woofwear boots which cost a packet but can be worn to ride in if it is REALLY cold, although I prefer her not to, and can be worn round the stables and for dog walking too. I would have a pair myself if I could get my incredible hulk calves in them 