Is it possible for you to weigh him? I imagine they have a pet scale there with more specific weight. But even if you have a regular scale, you stand on it, then stand on it holding Jack and subtract your weight, that could give you some idea? If we're talking just grams or ounces of weight difference, that wouldn't be exact enough though. (I've used a kitchen scale to weigh my guinea pigs, but I imagine a cat would be too big for that!)