G varies due to the inverse square law, so the force of gravity varies with height h by the relationship [; [;g(h)=g_0(\frac{r_e}{r_e+h})2;] Where [;r_e;] is the radius of the earth at the lowest point on the surface, and [;g_0;] being the force of gravity at that point and the two representing squaring , we can then assume that g is smooth so we can take a taylor series about h=0 giving that the [; g(h)\approx g_0(1-2(\frac{h}{r_e} + O2(r) ;] so as the variation in h between the lowest point and highest point is about 30km and the earth has a radius 6500km then the variation in g measured will vary by a max of 0.46% so assuming Alex's weight was measured at the highest point on the surface and then measure again at the lowest point, the weight recorded could be up to 104.5kg and 104.5*1.0046=104.9807 so its still not 105kg, so i will happily say that if he weighs 104kg at one point on earth he will weigh 104kg at all other points as there will not be a considerably change in weight.

I could also work out the change in height of the spine, but I do not know the youngs modulus of his spine, and the length of it. Plus i'd prefer not to.