Sophie, my ds is 10 mths and his weight dropped drastically! At birth he was 40% on the charts, dropping bit by bit until now at 10 mths he is at 15%! My 4 yr old dd was the same. My ped said not to worry, she actually warned me before he turned 6 mths that his weight would probably drop off, most babies do because they get active and start to roll around and move around lots. If you are bfing to hunger (so, not by the clock but by hunger which should naturally be about every 3-4 hrs in the day and NOT to every cry and wimper) and if you are feeding solids until they show they are no longer interested, then it can be very normal for weight to drop off (according to my ped). I have friends that have plump babies, they bf, they give formula, and they SHOVEL tons of food into their los and pry their mouths open even when you can tell the lo is not interested in the food- this isn't good as they are force feeding. The best way is to read their cues and follow their appetite. On a side note, would you and/or your husband be slim? Los take after parents. BTW, my ds is 10 months and 18 pounds, 4 ounces.
My ped told us to increase weight in los under 12 months you need to offer extra bfing, not solids, and to increase weight in los after 12 months you need to offer extra calories in their solids. When my dd was a baby and I was concerned/worried about her weight she told us to go for the higher calorie foods- ie. sweet potato instead of squash, and for chicken- to use the thigh or back meat instead of the breast meat as it's fattier. So, go after the higher fat and higher calorie foods. I sort of do it now for ds, I have to be honest though, it doesn't make a difference. I am not so worried that ds is 15% on the chart, I got over the worry with lo#1, she's now 4 and slim. Dh and I are slim, our kiddies will probably never be chubby or fat!
I do a combo of some purees and BLW with ds just to make sure he is getting some food. I do higher calorie purees- sweet potato, lentils, meat, and I add avocodo to my veggie purees or fruit purees. Ds never gets food such as plain carrot puree, it's just not enough calories! For his finger foods I give him fruits, steamed veggies, breads, pasta, beans, we have not done meat as a ff yet because he has no teeth and he still gags on meat pieces. So, the ffs are really low in calories as they are plain and simple foods, this is why I do the purees. I also added an extra bfing session in the morning and it's a regular extra feed now but he is not as interested in his last bf because like you, it's too close to his solids. With all this I think he has not gained extra weight, I think he is just maintaining his VERY slow weight gain.
Los can actually follow BLW at 6 months, or as long as they can sit up so they don't choke. Contrary to what some think, los do not need any teeth! My ds is 10 mths and has no teeth, los without teeth learn to mash food against the roof of their mouths. Waiting to offer finger foods past 9 months may be harder on a lo as the prime age for ffs is 8-9 months because this is the optimum age for when they really learn to use their fingers to pick up food.
