If he ever overwhelms the present american anti-french feelings and chooses to have a look at our country, then I think he will spend a rather good time, if he behaves respectfully, and in any case nobody is going to bother him about his knife if he carries a SAK. Here, Leathermans are mostly carried by LEOs or moving craftsmen (plumbers, electricians...) but a big lot of people carry a SAK or a Laguiole in their pocket, and no one minds if you draw it in a public place if you need it for some obvious cutting job.
Policemen are very subjective when they control you, and if you're good looking and friendly, then you can even carry a small concealed fixed blade without any trouble. Just do no let a ka-bar hang at your belt if you're neither hunting nor hiking in a wild place.
As a last point, due to the importance of touristic income, there is a tolerance towards tourists, and they enjoy more freedom with regards to the law than french people do and for instance they won't get a fine if they're caught driving a bit too fast. But of course you represent your country when you are somewhere else, and thus must try not to act on humiliating manners. This seems obvious, but I see really many foreigners who have not understood this.
May your brother have a nice trip !