One question to ask someone to keep a conversation going is "What do you do for a living?" while another question asks, "What is your family like?" Projects and hobbies are also good conversation topics when you want to keep conversing with someone, so ask the person what he likes to do in his spare time.
Another good conversation-starter is "What is the best part of your job or business?" People like to talk about themselves and what they know, and this question allows your acquaintance to elaborate on the part of his life that he knows best. People also like to talk about how they got started in whatever it is they do. Ask "How did you get started in that business, or why did you go into that particular business?"
Life has challenges, so ask, "What is the biggest challenge you face in your job or in your favorite hobby or pastime?" Because this question is more serious, wait to ask it until after you have talked to the person a little bit.
If conversation lags and you really want to keep talking, ask, "What is your favorite food?" You might even learn a new recipe or about a restaurant you have not tried yet.