My understanding is a stone = ~ 20 lbs?
Under that understanding, it's not a big deal. A 50 lbs difference, if your frame is the type to show that sort of weight gain (or loss) clearly? That should be mentioned. 100 lbs? Definitely should be mentioned. 20? People's weight naturally fluctuates at +/- 5lbs just due to liquid intake. But if you're really worried about it and have a cell phone that takes pictures, snap one and send it to him. I wouldn't worry about it, personally. People always look a little bit different than they do in pictures*.
* I say this with the caveat that if those 20lbs were somehow crucial to your current appearance, i.e. it changes it notably and kinda seriously such as someone who's 5'0" going from 100lbs to 120lbs, then a snapped picture probably wouldn't be a bad idea. Or a simple mention in an email. But if you're 5'4" and you're going from 140 to 160? The change in appearance is probably obvious only to you.