My twins were exactly the same and have on just started sitting properly over the last month or so. I was a bit concerned too because DS was sitting from about 5 months. I checked with the health nurse and our paedetrition and both said absolutely not to worry, especially if all other milestones are being reached. My health nurse said that when they were crawling, you would find they will rock back and eventually one day they will realise they can sit on their bottom, and that is exactly what happened. They also seem to have better balance - I remember DS used to topple over a bit when he started sitting, or get stuck. My girls prefered to sit up on their knees and were doing that for ages. They almost look funny sitting properly now! It was kind of weird that they could crawl, stand up and walk around the furniture before they could sit up.

Your DS sounds perfect and like he is doing everything else like a little champion. We would sometimes prop the girls up with pillows but it was normally more hassle because they would slide down and wriggle around. The hardest part was when other people held them and would go to put them down, and try to sit them on their bottoms and they would fall over