Perhaps he did find coins on Oak Island. He lived in Chester (perhaps he found them there?) He also owned Hook Island in Malone Bay (now THERE is a pirate treasure sounding island). But that's private and you can't dig there . . . so lets ignore it like most do.
I have no problem believing instead that he came from VERY humble roots, but turned his Revolutionary War land warrant (He served under General Clinton in NY) into good farmland and, through hard work over 20 years, when he moved to Chester, Nova Scotia - was already a successful farmer and businessman. Why is it so hard to believe his "fortune" was earned rather than found? Jealous locals?