My 18 months son not able to walk independently & won't response to us when we call his name

My son 18 months and yet to walk independently. He need to hold something then only he willing to walk or stand. I have tried to hold his hand & walk, but after few steps of walking he sure will sit down and started to crawl again. Another issue is that when we always call his name, he won't response to us. I am so worry for him.. what should i do..