Is he under developed?

I’m babysitting my friends baby that will be 6 months old on the 14th. I have a 9 month old of my own and I assume he’s advanced because he’s already standing up on his own for 10-15 second periods, rolled over at 3 months, sat up unassisted by 5, crawled by 5 months, and pulled to up to stand by 7 months and I see her child and he can’t even properly roll over completely. His arm gets stuck under himself. He can’t sit up unsupported. He watches tv. He’s a good baby, but he’s very laid back and under stimulated it seems. He doesn’t laugh. He is content with laying on his back just kicking and holding a sensory ball. I know every kid is different but he seems to have the personality and ability of a 3 or 4 month old. Maybe I’m being to hard on him but I am starting to think she will just lay her baby down and not play with him. She seems to let him watch tv all day. Is this normal for a baby that’s about to be 6 months old?