Keep warm in winter? It's simple! Here's how:

Wear Layers

Wear clothing in several layers rather than one thick garment: long underwear, warm shirt and pants, sweatshirt, sweater, windproof winter coat. Layers are warmer, plus they let you control your temperature by adding or removing layers. It's better to stay slightly cool, which helps your body adjust to low temperatures and keeps you dry.

Wear a Hat

Your head radiates enough heat to make your feet cold! Keep your head covered. If your feet are cold, put on dry socks AND a hat.

Change Your Clothes

Bring at least one change of everything and bring several pairs of socks. Tiny amounts of moisture in your clothes will make you VERY cold at night, so change ALL your clothes when you go to bed.

Long Underwear

If cold you need it, especially the bottoms. You can use a sweatshirt or turtleneck for the top.

Footwear

Winter footwear should have removable felt liners (wring them out when they get damp). Footwear must be roomy for extra socks (if it is too tight, circulation is reduced and your feet will get cold). Sneakers are unacceptable!

Mittens/Gloves

Keep your mittens or gloves dry. (if very cold, Mittens will keep your hands much warmer than gloves.)

Water

Dehydration is a special problem in winter!  In dry air and high altitude it will zap you before you know it. Bring a full water bottle on EVERY outing and drink plenty of liquids.

Sunglasses

They are helpful on bright winter days (and they look cool).

Pants

If in cold and Snow: Wool is best (try the army surplus stores). Avoid cotton. Jeans are worthless, and are unacceptable as Snow winter garb!

Keep Dry

Brush snow off yourself before it melts into your clothes. NEVER stand by a fire. Fires overheat you and make you sweat.
 

How to Pack

Put all your clothes in waterproof stuff sacks (which you can easily make from scrap nylon). Put your sleeping bag in a trash bag, then into a nylon stuff sack. If your sleeping bag gets wet, you'll have to share a bag with a buddy, which is pretty tight.
 

Sleeping Bag

This should be a good, machine-washable, four-season bag, weighing four to five pounds. The BETTER artificial fibers are preferable to down because they cost less, are easier to wash, and provide warmth even when wet. A Scout can use a blanket roll until you can acquire a good bag. He can also augment his bag in winter with an extra blanket or two.

Sleeping Bag Liner

Buy or Try making a simple sleeping bag liner out of polar fleece. This adds warmth, and the easily-washed liner collects dirt and sweat instead of the bag. Cotton one are also sold.

Sleeping Pad

A pad is useful year round and is essential in winter. Twice as much body heat is lost into the ground as the air! A pad insulates better and costs less than an air mattress. Buy a closed-cell foam (such as Ensolite; this is waterproof and costs less than an open-cell foam such as foam rubber, which require a waterproof cover). Self-inflating pads (like Thermarest) are nice for adult campers, but they are expensive and require care.
 

More on Footgear

For warm-weather outings, hiking boots should be the lightest weight and least expensive pair you can find that are comfortable and have lug soles. Because of a boy's rapid growth, you don't need top-quality boots, but proper fit is vital. Hiking boots should be comfortable right from the start (never buy boots that need to be "broken in"). Sneakers are acceptable only as a backup to proper hiking boots, and sneakers are never acceptable in winter.

For winter camping, you need warm boots with removable felt liners. The "Sorel" brand is a good choice for adults and older Scouts whose feet are not growing fast, but younger boys can do fine with less costly snowmobile or moon boots, as long as these have removable felt liners.