Can baby skip crawling and go straight to walking? if he does go straight to walking does he need to go back and learn to crawl?

Yes, No: It is not uncommon for babies to skip crawling and go straight to walking. This happens particularly with overweight infants. Once walking, they don't need to necessarily "learn" to crawl. They should be able to do that later on, as needed, particularly as they slim down from being more mobile.

Yes.: A baby can learn to walk before he crawls. He does not need to "go back". As long as children have lots of opportunity to be active and play outdoors on natural terrain and are not ushered into sitting still in chairs, they will develop all the walking, jumping, running, climbing, crawling, stooping, crouching, squatting skills that humans do need. They need help with swimming and bicycles.