If my DH threatened one of my children with a weigh in like that, I think he would be my ex-DH pretty quickly.
Bonsoir - the way your DP is choosing to deal with this is very worrying - from what I have read, if you are serious,(and given the gravity of the subect, I will assume you are) - he has some major issues with control (of you, his Ex-W, and his sons) and using weight as one of his weapons.
I read you have a small daughter - I hope for her sake she has a) not been privy to the recent discussions, and b) her father never humiliates her or threatens her if she gains a little weight (and in perspective of growing teenagers it is a little weight!)
It is true that Parisiens are extremely weight conscious (not to the point of directly asking one's weight though) - my 10 year old, skinny as a rake, daughter comes home from school full of silly ideas regarding weight - it worries me that she is beginning to have some body issues (i.e she thinks she is fat - for her age she is barely on 25th percentile!!) - and this is coming from external influences only in Paris (and we have not lived here all her life). We spend our time rebalancing her persepctive on the issue
PLEASE Bonsoir, for you daughter's sake, stand up to you DP on this - treat the weight gain as calmly as possible, help the DSSs lose it if that is what you all feel is best, but DO NOT humiliate them with weekly weigh-ins. I think you are setting yourselves up for some major rebellion by these boys later in their teenage years if your DH continues on this very controlling path.