I am also a bf mama, he is similar as you.
My bb is also around 9kg.
Don't worry, so long as he is playful.
The growth will slow down a little bit.
I don't think that being fat and being puffy means healthy. Each bb is different, and they have their own growth.
My girl is still on full breast milk. She is also not fat, only around 19lb. She has milk 3-4 times a day now but my milk volume has dropped significantly, like just now before coming to work I can only pump 3+ oz :-( since I fed her this morning at 8:00am. Luckily I have some reserves....
However, I think it's ok for her to be not as chubby as before, as long as her weight at age 1 tripled her birth weight and she's active and healthy. My boy is also skinny, he's 4 and only 31lb!!! So I'm used to hear similar comments
Weight is not very important, so long as he is still in the normal range.
For my bb, he is only 25% to 50%.
Just grows up naturally is OK, so long as he is playful.
Fat doesn't mean healthy.
Some mama even needs to put on diet for the little child, because the bb is too fat.
I still think that breast milk s the best, though needs a lot of effort and of course very tideous.