Probably better to estimate its volume, then multiply it by the density of water.

If Octuarga is resonably spherical, and his height is about 62 metres, then his radius is 31 metres. Volume of a sphere is 4/3*pi*r(cubed)

Plugging the numbers into that gives me approximately 125000 cubic metres (which is different from what I had last time, since I didn't notice how spherical he was and did it differently)

Since the density of water is pretty close to the desnity of flesh, we can say that since 1 cubic metre of water = 1 metric tonne, he weighs about 125000 metric tonnes.

Soooo yeah. You underestimated a little.