DS3 will only sit alone momentarily (max about a minute) before he falls backwards. I'm not sure whether he's doing it on purpose (he often laughs when he falls down) or he can't help it, but he prefers to spend his time lying on his back. He wasn't able to sit up at all until about a month ago. I should point out he has a very big head (above the top centile) and is also a very tall baby with a long back so sometimes I think the weight of his head may be pulling him over. When I put him on his front, he rolls straight back onto his back and has even started to move around on his back. He can do a 360 degree turn in a matter of seconds. DS1 and DS2 were also quite late with gross motor skills (DS2 didn't walk till 20 months and sat up at 9 months). So, should I be worried about DS3? His small motor skills are good and he can clap and wave on command and can also say a couple of words so I am not worried in that respect.
My dd took longer to be able to sit up and she'd fall backwards. I borrowed a thudguard hat & this helped her not get hurt by the tiled & wooden floors. She's now 2 and is running about. Could you ask your doctor to be sure everything is ok?
My dd did the falling backwards thing until about 13 months due to a weak trunk. I also wasn't sure whether she was falling backwards on purpose at times but always put cushions behind! If late gross motor skills are in the family he's probably just following the same pattern but always worth getting him checked out just in case. My dd has physio for her trunk muscles which has been helpful!
I saw my health visitor about it and she said given that my other children have taken longer to walk/roll etc, then it is probably normal for my family. I don't want him subjected to loads of tests and paediatrician appointments if I can help it. Good tip about the hard hat as I have wooden floors!