I have a massive bugbear with weighing, it causes an awful lot of unecessary worry I have found . Babies are supposed to grow slowly, decreases the chance of obesity and heart disease later on, the chubby baby being healthy idea is a myth and is a result of formula, made from cows milk and cows bodies grow faster than brains! We are the opposite.
Do you know what charts you have in the red book? Do you have the new breastfed baby chart or the old one which contained formula fed babies in it's statistics? That can make a difference.
Personally with my children I have only weighed them to see if we need to get another car seat! I think doing it too often causes worry and should only be done if there are other causes for concern, like too many dry nappies or a drowsy inactive baby. Mother's instincts are pretty good if listened too. If you feel he's alert and doing well he probably is. Sometimes just weighing before or after a poo or feed can alter things. Perhaps the scales are different or have been moved and jogged. If it's a one off blip that could often account for this. But as the say watch the baby, no the clock, scales or calender! They are all different.
This website has some great advice regarding dieting/exercise and breastfeeding, in fact it's the best site for breastfeeding anyway.
http://www.kellymom.com/nutrition/mom/mom-weig htloss.html
Have you called a trained counsellor at all? HV's don't always have the time to keep up to date with the latest advice with regards to breastfeeding and they may have some more ideas. Perhaps keeping baby on one breast longer before switching can help him get more of the higher calorie hind milk (milk like a hot tap, the longer it runs the higher the calories, take baby off to soon and all he gets is the "drink").
It's something you may wish to talk abo