L.P.
My stepson did not walk until he was 17-18 months. He also got around by scooting on his bottom. In fact he didn't even pull himself up until he was 16 months. But as soon as he figured out how to do it, he was gone. I wouldn't be too concerned, especially since atleast 2 doctors have said to not worry about it. But I will tell you that the reason that my step son did not walk or even crawl was because of the lack of attention and effort that his mother put in to him. She just sat him on the floor as soon as he could sit up and let him play with all by himself. She did not put him on his tummy or anything. I am not saying that this is the cause to why your son is not walking, just letting you know. I think that all kids develop different and I really don't think of this as abnormal. He will do it when he is ready.
Good Luck!