If you REALLY want to know his weight, and if the vet isn't far from home, just call in a few days /weeks to see that the scale is working and take him in. I do this all the time with my cats. I go in the morning, when they don't have appointments as they are taking in the surgeries for the day and working on those. So the exam rooms are just sitting there empty. When my cat Sparkle had to lose 6 pounds (!), I used to joke she was in for her Weight Watchers Weigh In! Just a thought.