First, your idea of "walking distance" and someone else's (especially WDW's) may be completely different, and is likely to change/shorten as your trip progresses. If you really are trying to avoid walking a ton, staying at one of the smaller resorts and/or in a Preferred room could help with that. Be warned that the moderate resorts are generally large or spread out (or both) and even a "Preferred Room" may not save you much walking there (looking at you, CSR). There is a reason my family adopted the saying, "My feet hurt like Disney." You're correct that one or two days of that much walking is not likely to be a big deal, but beyond three or four, you're gonna feel it. Totally worth it though.
Every. Time.
My family of five loved the theming and convenience of POR but did not enjoy the lack of both at CSR and will never stay there again. (Many others swear by CSR and will stay there every time if given a choice). We adored the Lion King Suite at AoA we stayed in on our first trip because it had a bedroom with king bed, two full baths, and both a fold out couch plus a table for four that converted to a murphy bed as well as a kitchenette. AoA is a value resort and if you're wanting to avoid a lot of small kids it might not be a good fit. I would definitely encourage you to take a look at the family suites in the values, though. The theming is FANTASTIC and AoA does have a Skyliner station. AKL has wonderful theming as well and we have absolutely loved visiting when we've had dinner reservations there. Our next trip we'll be staying at BC (or possibly YC) as both are walking distance to IG of Epcot and have boats to access HS, though IG at EPCOT has the Skyliner, which we can also use to access HS. Neither has particularly delightful "Disney" theming from what I've seen (again, dinner reservation), but they are both themed well for what they are intended to be. I cannot give any input on GF as I've never visited nor stayed at that resort.
Research carefully the fold down beds as most are smaller than a regular twin bed. Assuming your son is a typically-sized senior boy he will likely be uncomfortable on one of these. If he is taller or built larger than typical, he will almost certainly not be comfortable. Though inflatable beds can be heavy, he could very well be more comfortable on one of those, depending on what the room has available. As long as you don't exceed the room occupancy (shouldn't be an issue with just three of you) having an inflatable mattress isn't an issue, by my understanding.
Last I heard, WDW was not moving luggage between hotels for split stays. If that hasn't changed, please consider that staying at three resorts means two transfers between them and may result in significant time and hassle getting everything moved from one to the next. As others have done, I would recommend no more than two hotels for that short a stay, but ymmv.
Buses can be a VERY long walk from the park entrances and the bays the different hotels are assigned sometimes change, so there is no hard and fast rule re: which resort will have the closest bus stop at the park. My least favorite part of visiting WDW is leaving the park and realizing our bus stop is the furthest away, which seems to happen more often than not. We have had to wait for subsequent busses more often at a AoA, but we also had to wait a few times at POR and at CSR. During the Holidays I think it is likely you will have to wait for another bus more than once, both going to and especially coming back from the parks.
One more thing I always recommend for WDW: Bring at least two pairs of good walking shoes for each person. Popup showers are common and overnight is NOT long enough for tennis shoes to dry out in your room. If you have a laundry drying it probably would be nbd, but using a hair dryer to get them dry enough to use the next day is a stinky, stinky nightmare.
