Can you tell me if someone who is coming to the US on a 6-month B-1 visa needs a return ticket or onward journey ticket when he arrives in the US, or can he buy a return ticket after he gets here? We're not sure how long he'll be staying, so if we were to buy a return ticket and then not be able to use it, it will have been a waste of money. Thanks.

How can you convince the immigration officer that you will return home? B-1/B-2 is visitor's visa, NOT immigrant visa. I would say he's a lucky one if he gets 6 months. Better get a round trip ticket and leave the return date opened so you can reschedule any time.