Like Ezra, who has already answered, I wouldn't say he works in business. I would specify which area. For a general remark I would say, He's a businessman." Or maybe, "He's in business," but that has another meaning so could be confusing. The other meaning is this.. supposing my friend and I are having problems getting the car started... and eventually we succeed! Vrroooom! The engine starts! I might smile and say, "Hey! We're in business! Let's go!" It means to succeed/solve a problem/to be able to proceed/get going... that sort of thing. But really it's not confusing as the context will tell you if he's a businessman or he's got the car going!