His mind is developing. Baby may sit up on his own for longer periods. He can reach for things without falling, because he doesn't need to prop himself up with his hands. His neck and back are getting stronger, so he sits up straighter, too. If not, he probably will soon.
Some babies never crawl.
They scoot on their bottoms or roll. Then, they walk when they're ready. Here are some games your baby may enjoy. The baby can try to help you find the missing one. Let your baby watch you as you hide a small toy under a cloth or cup, or in your pocket.