Taffers, if it makes you feel any better, G has no desire to sit, either. I didn't ever have to do adjusted age for the twins, since they were born at 38+2, but they'll be 7 months on Thursday, and he isn't even close to sitting. His twin sister, on the other hand, has been sitting unassisted for weeks. She also rolls both ways and sleeps on her side. He's perfectly happy to lay flat on his back, and he never even moves his head when he sleeps. (He's also got a pretty flat head, too, but so far, our ped hasn't been concerned.) He's the same way as your DS-pushes against you the minute you try to get him to sit. The only thing he *does* like to do is stand.
R+P 11.09.02
C 04.06.09 + G & J 12.19.11