M.S.
Have you tried a pediatric sinus wash/rinse with your son? He's old enough to try it, and probably the squeeze bottle is easier to handle than the neti-pot, because you can control the flow and your son won't have to tilt his head. Maybe that will help him to clear his nose, and pretty much it's instinctual to blow out the excess saline in the sinus cavities. Just a thought, good luck!