Now that I have come with a new topic, I realize I have not updated this topic and all of you who helped me, sorry about that and once again, thank you for your help and your concern.
Luckily I have finally discovered the "Follow this topic" button and such lengths of time before my answers will not happen again.
In the end, after multiple try-ons, it ended as you suggested, putting the effort on th upper thighs. The centerback seam dart was not of much help indeed and obtaining a perfect fit on the kidneys helped to give the impression of a more horizontal lower back, and thus a rounder bottom.
All the problem ended in finding how much material to take off at upper thighs without creating too much tension and loosing the fit of the lower leg.
My client accepted to opt for a slightly stretch fabric and in the end was happy with the result.
Too bad he refused to have pictures taken of the result but maybe I will do one for myself after I think about some improvements for difficult physiques (as I am very erected, flat-butted and usually walk and stand with feet very close) and will post pictures then.
This is thanks to you all that i could make a man happy and feel good, and I hope I can be of help in return some day soon (although I must be of the youngest on this forum ^^).