There's no real time frame for babies to sit up, crawl, walk, etc. Each baby has his/her own pace. My nephew didn't sit up until 8 months, and he started walking at 16 months. So don't worry about it. Let your little guy take his time. ;-)

I have 2 children and I know how hard it is not to try to compare them with other peoples, sitting up comes around 6mths-9mths so if he is not yet 6mths there is no need to worry! He will sit when he is good and ready. When my first son was a baby he took forever to walk, everyone elses babies at the same age were all running about and he used to just sit there looking at them, he eventually took his 1st steps at 15mths but believe me this ended up being a blessing , once babies are up and about they are into everything and you hardly get a chance to sit down yourself!! Lol!