He's been able to be in the sitting position for a while, but needed somebody supporting him or have the Boppy pillow surrounding him. He still tips over eventually, but I'd say if you can sit unassisted for five minutes at a time without tipping over...you are a sitter!
Now I have no hope that he will roll over because he will always want to be sitting. I guess that will come when he learns to...gulp...CRAWL!
He is growing up so quickly but I'm enjoying him and each stage he is at so much. He is very interactive now, which is fun, especially for the Bigs.