Over the past 6 months or so, I moved by base weight from 20lbs to well below 10lbs. I suggest you start by buying two books. "Trail Life" by Ray Jardine, and "Lightweight" by Ryan Jordan. A lot of the information they present is available on the Web, but it is nice to have all of this stuff in an organized one source location.
Regarding a poncho or even a poncho/tarp. These have been my favorite rain gear and even shelter for over 40 years. But do some research, the methods are not for everyone. For you and your son, a 8' X 8' tarp would work, and I think it is better than a tent. But others disagree, so do your homework here.
Going through your list, I will skip items others have mentioned. Other than the pack. Assemble all your gear, and then find the pack. What I would do is figure out what capacity I need, then go with a ULA, Gossamer Gear, or one of the other lightweight packs. I am really impressed with the offering from ULA, and bought one of their packs a while back. I might even take all my gear to REI and try some of their packs to get a feel for capacity. I think REI carries some of the GoLite packs.
Insulation - I have a Patagonia Down Sweater, and it is awesome. But at 1/2 the weight, my MontBell Extremely Light Jacket keeps me warm down to 20F, with a shell worn with it. I have not used it in colder weather.
Sleeping bag - Western Mountaineerg Ultralite (20F) at 29 oz, or Versalite (10F) at 32 oz are much lighter than what you have. There are other manufacturers with similar ratings and weights.
Trail Runners - I was surprised how well these work. Of course you need lighter loads to use them.
Tent - Go with a tarp or tarp tent. But do you research, as they have their limitations and you need to know how to use them.
Pads - I now use the 10 oz BMW Torsolite, or the Gossamer Gear 3.5 oz Gossamer Gear NightLight foam torso pad. I can get a good night's sleep by site selection and using my pack under my legs. You will have to do things diffently though, for snow.
Everything else - a lot of little items can be reduced. I would review a lot of the BPL members gear lists for ideas.
You can easily cut your base weight in half if you want, without giving up comfort or safety.