A vote for belted trousers

[Merc]

young frank sinatra was also too awkward
he was too damned skinny--the ears stood out too
i think the guy looked best from his late 30's into his 50's
even with thinning hair and later toupee

[Merc]

also--i think thats a bad choice of suit cut (in that photo) for someone so thin
exaggerates it --makes him look like a stick with no chest at all

[hectorm]

...on the other hand - put Agnelli's suit on another man who lacks the attitude: they will lose their charm instantly, even if they fit and everything else

Agnelli´s attitude is unique and he seems to have developed it in his teens.
I can´t imagine anyone else wearing his tie pulled outside his crew neck sweater....

...I think that´s a bad choice of suit cut (in that photo) for someone so thin

Well, after close scrutiny, I tend to be of a different opinion and if I may try to convince you otherwise:
The broad peak lapels and padded wide shoulders of the jacket balance off his larger head very well (a la Cary Grant) and add certain needed weight to his thin frame.
The flowing trousers are in line with the width and amount of fabric of the jacket and negate what probably are skinny legs.
The DB jacket should not be worn open, thus showing so much shirt and pinched fabrics. But that´s not a problem with the cut, and is part of his attitude caught rampant in the picture.