Stubborn Son Won't Walk: UPDATE- Thanks.

My son is 13 months. He is not walking on his own. He will only walk when assisted or if he's holding on to furniture. I have 2 friends whose babies started walking at 9 and 11 months. My son was 10 weeks premature so I know to expect some delay, but it still seems like he should walk unassisted. Is something wrong with my son? How can I encourage him to walk without help?