MamaG - my Ds2 is the same age and at a similar stage developmentally too - he can sit for a few seconds then topples to one side.
He too is bfed and feeds loads, esp at night grrrrr.
I'm giving him a small amount of puree, baby rice, porridge or similar once a day, and he sits in his highchair at the table with us when we eat, and I offer him finger food.
The purees hardly go in, though I had marginally more success with baby porridge yesterday. The finger food gets held, occasionally sucked for a second, then dropped on the floor generally.
I plan to carry on like this and see if he seems to show a preference for purees or finger food, and that will make my mind up for me as to whether to go in the BLW direction or not - though I know I'm technically doing it wrong already by offering purees.
I don't see the point in offering more than one puree meal a day until he actually eats a bit more though - atm he spits most of it out again, but he gets offered finger food 3 times daily when we eat. He's yet to eat anything but seems to like holding it and squishing it etc
HTH