That doesn't sound like much of a delay to me. My oldest, who was born on time, didn't walk until about 14 months old...basically, was at the same place yours is now. My second is turning one next week, and walks well. They're all different.
I don't think 15 lbs is too small, either, for a preemie baby. My youngest turning one weighs about 24 pounds, but chances are she's just a lot taller and has a bigger frame. I know several one year old babies who are around 17-18 pounds, so she's not so far off from that.
Good for you for doing early intervention, though...I honestly don't think you have anything to worry about, but it never hurts to check. Prayers for you and your family!