My almost 19 month old STILL isn't walking?

My son is almost 19 months old now and STILL isn't walking. I think he just seriously doesn't want to. He's very smart. He can say a ton of words and say some 3 word sentenses. He can walk fine if he's holding someone's hand. He's a preemie (6 weeks early). The pediatrician said he'll probably start walking when he goes to day care in August because he's now watched at our home by a family member so he's not getting too much stimulation and motivation to walk. Still worries me because he's so late.