DS is 8.5 months. He sat up at 6 months but gets bored being in the same place for more than about 3 minutes. He can't crawl yet though and it's starting to worry me. He rolls across rooms and can go about 2 metres in one go and he can also change direction. He's recently started bending his knees sometimes when he go to put him down on his tummy. He stays in the crawling position for about 30 seconds and then collapses on his tummy again. He's very inquisitive though and I wonder whether he doesn't bother trying to crawl because he gets around by rolling instead. He's also able to stand while holding on to the side of his cot or a chair. Please reassure me that he's ok!