The word "benevolent" means something like "kind" or "generous." A person who is benevolent is one who acts out of a desire to help others rather than simply acting for his or her own gain.
In the sentence that you give us, this definition works well. If the old man was benevolent with his fortune, it means that he used it to help other people. Perhaps he donated it to charity or perhaps he set up a foundation to give scholarships to worthy students. At any rate, he used his fortune in generous ways, not selfish ways.
"Benevolent" means kind, generous and unselfish.