Hi Chicane
The most important thing is that he can sit up, even if it's with assistance. Our LO could sit up in our laps but would topple over at six months old. It's all about the neck muscles, if he can hold himself up he has the strength to swallow.
I've never fed a single purée, I didn't see the need to so I can't really offer advice here. I know some people mix the two as they are worried that their LO's are not getting enough food in, but the way I see it BLW lets the baby lead the way.
I think it is really important to include your LO's in the family meal, it is the best way for them to learn about everything from how to eat to etiquette. Our LO is nearly 19 months old and has been able to eat with a fork and spoon for months, she is learning all the time from watching us. I'm not sure if she's advanced in this area over spoon fed babies as I have nothing to compare her with, but her pincer grip developed weeks ahead of others.
Is there any way you could adjust your routine to fit him in around you? We didn't have her eat with us at the start as she took so long, but after a month or two we changed our meal times to include her.
HTH
-x-