Re: Delayed walking and development question..
I wouldn't worry about the walking yet, but have you brought up the sitting part with your ped? At 18mos he should definitely be able to get into a sitting position and be stable sitting on his own. My 31 weeker had PT because she couldn't get to a seated position at 11 months.
Kristie mom to two girls, ages 5 and 8, and two boys ages 3 and almost 1!