Absolutely yes, include him. You might need to break your party up…try for 4 & 6 at the same time or close together. When you check in, ask to be seated together or as close as possible. Most restaurants don’t have a lot of spots for bigger parties. So it might be easier to get into places with the smaller parties.
But everyone counts, no matter how old. This is even stated on the WDW website when you make reservations. Being able to add the extra person depends on the restaurant‘s popularity & it’s capacity at the time of your ADR. Restaurants absolutely do turn away extra guests if they are at capacity. This question comes up pretty much weekly, so it isn’t a unique situation.