Layering clothing and a poncho are great. Weighing almost the same as a poncho are those little tube tents but they require some cord or poles or making poles out of branches. Might be an option for some. The Gatewwod Cape by Six Moons Designs is the ultimate in poncho/shelter design in my opinion. I love mine. 10 oz +/-, multifunctional, practical and extremely well built. (Maybe a bit much for an essential kit as far as $ and space/weight is concerned but well worth it.)
It always seems like it’s around 2am when it gets really hard if you are in harsh conditions without adequate shelter. There’s those 4 or more hours of dark cold ahead and you are already shivering. Personally, this is when I appreciate the benefits of a good fire. I used to do quite a bit of survival hikes, which is basically what you are posting about here but with a destination to make it to by sometime the next day, and some of those nights it was much better to hunker down with the minimal gear I’d take rather than keep hiking thru the night. The warmest/driest lightweight/small thing I’ve tried is an emergency bag. The Mylar sleeping bag. One that is long enough(84”) for you to get fully inside so you can go sideways and keep the rain off if you have to. Seemed to work better than the emergency blanket to me. So well that I keep it in my kit to this day. Don’t get me wrong, it’s still miserable, but … maybe less so.?. This and an emergency blanket as a lean-to to keep the rain/wind off and reflect the heat of the fire at your back, is the lap of luxury for emergency overnight shivering. I think these 2 things with some cord are about 8 or 9 oz. and tiny. Fit in your medic kit which is a good place to have um, hypothermia, shock, etc.

And, +1 on hand warmers and small CCF pad. I always have a small pad with me, reflective on one side.

4evrplan – Good work with your son. I found it extremely helpful to have an extra emergency blanket to give my kids to, uh, “work” with. They had a good time destroying it and learning it’s limitations. The one thing I would stress is that your boy know how to use every piece of gear you throw in there.