I would pick whichever one is most appealing to you. If 2 weeks is break, then he is only missing one week of school, not 3. Also, in our experience, the week before Christmas break in grade school they don't do a ton of real academic work anyway because the kids are so wound up for Christmas it is hard to get them to focus. It is likely that he would miss more academic stuff in November. The one drawback to December is that if his school has holiday parties, then he will miss that, and he will also miss whatever other holiday activities they do. For DD's school, they have their Christmas shop that week where they take money to school and then they can buy gifts for all their family members at the store. I don't know what different activities his school might have that he might miss out on. At our school, they seem to fill that week with activities that are more "fun" oriented like art projects, learning games, hands on experiments etc. because they know the kids are too hyped up for much else. DD is also a performer, so December is pretty heavy with Christmas performances- you might think about what he will be missing out on for his out of school activities too- sports games, performances, Christmas parties, etc.
That said- WDW is wonderful during the Christmas season and if there is nothing that he really will be upset about missing, that would be my choice and you seem to be leaning towards it, so I would say go for it! We went between Thanksgiving and Christmas 5 years in a row and absolutely loved it. We then switched and went in October the last 2 years. Fall is fun too, but going the day after Halloween, you will likely miss the Halloween party, and the decorations will be in transition. The parks should have most of the Christmas decorations up by the time you leave- but there will be resort decorations and park events that probably will not have started. You were there the 2nd week of November this year, so you know what to expect. Pretty much the same as you got this year, but maybe with not quite as many of the decorations completed. Based on our experience, the weather in December is crazy. It might be between 80 and 90 degrees for the high, it might be 50 degrees for the high, or you might get both in the same week. We have started off the week day and night in shorts and t-shirts and ended the week in sweaters, gloves, and hats. The weather changes fast too, so you can't trust the forecast. Also, if swimming is really important, you might think about that. We typically don't swim on our December trips. We have had days where we could have, and days where there was definitely no way. When we go in October, it is pretty much always swimming weather.
You have plenty of time to decide. The year we switched from Christmas to October- I had both trips booked with all my ADRs until about 2 or 3 months away when I had to commit to airfare.
