Recipe: Sugar Cubes & Royal Icing work well

Susan, I suggest you let him "go for it" with sugar cubes joined by Royal Icing. The sugar cubes would allow him to work on his castle in stages, and design the castle as he works with it.
Royal icing hardens completely (like mortar between bricks.) It is the same icing used to make gingerbread houses and the "hard" flowers etc on wedding cakes.
The ingredients in Royal Icing are edible, however, the icing is not particularly intended to be eaten (unless you have very good teeth, or a dentist nearby).

ROYAL ICING
1 Med Egg White
1 Cup Confectioner's Sugar
Lemon Juice(a little for flavoring) optional
Combine ingredients in a very clean bowl and beat using a slow to medium speed until icing is completely smooth and "glossy."
Hints: Before making Royal Icing, make sure your bowl, spatula, and beaters are completely oil free. The tinyest bit of oil will cause the icing to separate and it will not be usable.
And, when working with Royal Icing, keep the unused icing covered with saran wrap pressed directly onto the icing. Any icing that is exposed to air will begin drying.
Have a great time. This sounds like it could be a fun, fun project. You could use ice-cream cones for roofing, or castle "crowns", candies for window trim, etc.
;-)