In addition to saving on admission you have to factor in discounts on dining, recreation, hotels & merchandise with an annual pass. When you return to Disney in March you can book a room only reservation for everyone and then buy your son’s ticket separately from Disney or an approved vendor like Undercover Tourist. You may have some trouble with the dining plan because you need an AP to add it–I’m not sure how that would work with your son having a regular ticket. But–you can purchase a Tables in Wonderland card as an Annual Passholder & save 20% on most table service restaurants. You might save more by doing it that way than buying the dining plan.