Happylady, If you requested a room with special needs (disability), you will be in bldg. 14. This is a fantastic location, next to the food court and main bldg. These rooms already have a trundle bed in it. However, if your son is older than age 8 or so, it will not fit him. It is pretty small, but adequate for a child. If he is older than this, I would recommend the aero bed. It can go in the space that the trundle bed would be and the trundle bed would just stay underneath the bed.
Best of luck with your surgery. You will LOVE this resort! Be sure to take some time to walk (or wheelchair?) it around for awhile. Also, take the boat ride in the evening to Downtown Disney, wonderful ride. Have fun!