If the guy asks me if I already got a passeport before my canadian one do I have to answer?
If you refuse to answer, you'll probably be refused admission to the US.
Does he have the right to ask me that question?
Yes. An alien applying for admission at the border to pretty much any country has very few rights to privacy.
(Btw in order to avoid any issues I might just asks/do whatever he gonna ask me)
A good plan.
I used to travel to USA with my tunisian passeport (green passeport with arabic letters)
Keep in mind that if they ask you about other passports, it's probably because they know you have another passport and want to see whether you'll tell the truth. Records of visas, entries, and exits are computerized and easy to search instantaneously. Your name and the place and date of your birth will be trivial to match. Even if there are slight differences, it's possible that they have access to Canadian databases that would link the new passport to your other passport.