When did your baby walk? How can I encourage my 14 month old son to try to walk?

My son is 14 months old and basically is very content cruising and crawling. He's a super fast crawler and efficient cruiser. I don't think he really sees a need to walk on his own. He can stand on his own for very short periods of time. He can say a few words, and has very good problem solving skills. He can figure things out pretty quickly. He just doesn't seem to see the need to walk. I do walk with him holding his hands, but he only will do it for a short period of time before he gets down and crawls. When should I start being concerned about his lack of walking and have him evaluated by a doctor about it?