Is Grandpa Dragon Completely Hollow?

Grandpa Dragon

Grandpa Dragon -  2M (6.6 feet) tall, Weighing 3 Tons.

No. He has additional supports built inside of him. 

We expected that possibly up to 15 people might climb on him at once (we've now counted up to 26 adults at once!). To make him robust enough, we first designed and built a "grid" made of 3/4 inch thick vertical "slabs" of Pal Tiya Premium. Much like those plywood laser-cut dinosaur models you see at Museum and Science stores. Each of these intersected/bonded about every 18 inches or so. His head, horns and neck down to the foundation were filled solid. 

Grandpa Dragon's internal structure (and Matt)

You can see the exposed tops of some of these 'slabs' in the picture on the right, next to Matt.

Being Craned into position at Wellington Zoo. Here you can see the 'reinforcing/foundation ribs' cast into his base between polystyrene triangles.

Our Engineer's report seemed to indicate "you could land a 747 on him" (not that he's the right shape to do that sort of thing on).