It's important to regard the various published developmental markers with flexibility. There is wide variation among children in relation to general development, and variations within the same child in rate of development of particular skills. Actually, based on the information you offered, it sounds like your son is doing fine. Keep in touch with his pediatrician if you have worries, but don't place excessive stock in specific time frames - they are meant to be general guides, not detailed road maps, of development.
Hi Vicky,
I am a mum of two and they are both very different, my son crawled at 6 months and my daughter is 11 months and she only started sitting up at 8 months, she then crawled about 2 weeks later. My niece didn't crawl until nearly 12 months and then started walking at 12 months. Children are all very different and I think sometimes,(although milestones are important), people put to much pressure on 1st time mums to make their child conform to the advridge, and after all who wants an avridge child, mine are both special in there own ways.