If we convert wailord's weight of 877.4 lbs into newtons (metric unit of force) we get 3904.43 newtons. Now, weight = mass x force of gravity, gravity is about 9.8, so we find that wailord has a mass of 398.4 kilograms.
I didn't bother trying to estimate the volume of a wailord, instead I decided to find out the volume of air that has the same mass as a wailord. I looked up the density of air, and found that at sea level it has an approximate density of 1.2 kg per cubic meter. density = mass over volume, so the volume of air with that mass ends up being about 332 cubic meters, converting that to cubic feet and we get 11,717.65 cubic feet. If a wailord has less volume, then he's more dense, otherwise we've somehow proven that something that doesn't fall towards the ground has a positive weight.
Well, we know the length of a wailord, so if we divide by that we get the area of the other 2 dimensions, which is about 246.3 square feet. This means that on average a cross section of a wailord would be less than 246.3 square feet (in order for it to be heavier than air).
So now let's do some approximations. From the only 3 dimensional images I've seen of wailord (being the gamecube games), I would say that his width is about 1 quarter to 1 third his length, let's take the larger one, and say he's one third as wide as long, so he's about 16 feet wide. And I'll say he's half as tall as wide, so that's 8 feet. So on average his cross sections are about 128 square feet, which is just over half that of the cylinder of air.
So in summation, a volume of air with the same mass as wailord is 11,717.65 cubic feet, whereas I figure a wailord has an approximate volume of around 6100 cubic feet. So he is clearly heavier than air.
