Once the last Halloween party is over on Oct 31 or Nov 1 (depending on what Disney schedules next year), the fall/Halloween decorations are removed and the Christmas decorations are put up in the MK. They will be complete by the first Christmas party which is generally a week after the last Halloween party. At the rest of the parks decorations will be installed gradually. Typically not much is done at Epcot until after the Food & Wine Festival ends and the Epcot holiday activities (Candlelight Processional, Holiday storytellers, IllumiNations holiday tag) will not begin until the day after Thanksgiving. At the resorts decorations and gingerbread displays will be installed gradually throughout November, but again, not all the resorts will be fully decorated until Thanksgiving.
If you want your son to see everything set up for Christmas you will need to wait until after Thanksgiving.