Winter clothing system for small 3yo?

I'd like to try some winter camping and take along my 3 year old son. He's pretty small for his age – 28 lbs and some 24-month pants just fall off of him. I suspect he's a mini-oven though.

Anyway, what kind of layers should I be looking at for him, knowing we won't be going more than a mile or 2 at a time? Snowfall is pretty minimal in this area and I doubt I'll take him out if temps will be below 15 F at night.

We'll just practice in the backyard a couple times first, since I'm new to backpacking. Well, no catholes in the backyard – don't need to start a bad habit!

I think I'll have him carry just a few small toys, a stuffed animal, and maybe a water bottle in his tiny backpack.