To understand BLW, you really need to understand a bit more about the physiology of learning to eat. (Apologies in advance if you know this)
Taking in very smooth purees is of limited use to learning to take other foods. Imagine you are drinking a drink through a straw, you basically suck it to the back of your mouth, you don't move it around it.
Now you can still do this (though it wouldn't be great table manners) with slightly thicker foods, like yoghurt. However, at some point, you are going to need to learn to move food around your mouth too. It makes more sense in some ways for the baby to learn to move things around their mouth as a first step and then work up to swallowing, rather than mimicking milk swallowing and then having to learn a new technique part way through weaning. So even with conventional weaning, you are generally advised to give finger foods alongside from six months to assist with this learning process.
When babies were weaned at 3 or 4 months (or, like me, 10 weeks 
), you needed very runny purees. With a much bigger baby you don't.
With BLW, or finger foods you give alongside purees, what you are doing is assisting the process of learning to move food around the mouth. They will swallow very little to start with, and the gag reflex is very far forward on the tongue. You obviously don't give anything very hard, but cooked carrot sticks, toast fingers, broccoli, cucumber, banana, etc are all good.
On volumes, you don't need to worry at all if your baby is demand bf (which I think I remember he is?). Just let him eat what he eats and continue to feed on demand. For example, I did BLW with both of mine. Sometimes I offered food twice in a day, sometimes once. They didn't much care. I added breakfast last as it was the meal they were least interested in.
HTH a bit.