What Causes Delayed Developmental Milestones In An Infant?

Hi, my son is one year old. He s so smart and I love him so much! However, he seems to have a problem with his right arm. He is left handed, and I understand that means he s not going to use his right arm as much however he s actually 13 months and doesn t crawl he can scoot around but he doesn t use his right arm to help. He will actually just let it drag on the floor and it scares me bc it reminds me of a stroke victim. That s actually exactly how he carries it. But don t get me wrong he doesn t act like it hurts him or anything he can clap his hands and such, I ve tried working with him but he still doesn t use it to crawl. He also doesn t feed himself? Which I found extremely odd. I d understand if he missed his mouth or struggled with a spoon but he literally doesn t put anything in his mouth, teething or not. Most babies his age that I ve seen are putting everything in their mouth.