I have heard that on average, a horse can comfortably carry 30% of his body weight. So if he weighs 1000, he can carry 300. It also depends on the breed and conformation, a finer boned horse or one with a long back may not be able to carry as much weight as a better conformed horse of the same size. What breed is he; QH, TB, draft cross? With no more information than his height, I would say that he could probably carry up to 325 lbs but I could be wrong.
I have heard the 30% thing as well, but like smrobs said it depends on the build and bone of the horse and it also depends on the rider. I horse will be able to carry a heavier rider that is light and balanced in the saddle versus a lighter rider that sits like a sack of potatos on his back bouncing all over the place.
Acualy it's 20%. Like say the horse is 900, its rider should be around 180. 1000=200, 1100=220 ect. This is mosty for seriuos riding like trail or a work out, not just a few time around the paddock. Though in some cases if the horse is thick boned it can hold more. Thin boned it holds less. A weight tape is a way to get a good idea of the horses weights.