Answer:
Advised him and tell him: You should not swear a lot, even if you are truthful, because of the words of Allah Almighty that says وَاحْفَظُوا أَيْمَانَكُمْ [المائدة:89]،
guard your oaths. [ Ma'dah 89]
It is prescribed for the believer to diminish his oath even if it is sincere. Because a lot of them may lead him to lie. It is well known that lying is forbidden. but if necessity or overriding interest calls for a oath, then there is nothing wrong with that.