I've had exactly the same problem. DS has always been in the 10th percentile for height and weight and we started BLS at 6 months but he wasn't particularly interested. It didn't bother me too much as I knew he still got plenty of BM but then at around 8 months he started losing weight (previously he was going steadily along the 10th %ile line). It was only a little but I couldn't help but doubt myself a bit. I'd always maintained that he is just a small fella who was born a little early, but now that he was losing weight I just couldn't justify relying solely on BLS.

The MCHN was actually always quite supportive of my choices and suggested introducing meat BLS style to put some weight on but DS just couldn't manage it especially as he had no teeth. So I started spoon feeding him one meal a day (mashed or finely chopped) and now his weight has improved greatly.

Now at 10 months we're back on track with BLS, he feeds himself mostly but we have the odd spoon fed meal for yoghurt, porridge etc but I try and encourage him to put the spoon in his mouth. He's still a little guy but very healthy,active and reaching some milestones early etc. The spoon feeding seemed to work as some gentle encouragment to show him that food was more than something to play with.

I'd say go with your instincts on this one, I know how hard it is to stay positive when people always comment on how small your bub is or you see them next to other babies their age, it also doesn't help that MCHN/doctors generalise so much with those graphs that do not take genetics into account. I would have continued with just BLS if DS hadn't started losing weight, I don't regret my decision but I'm so glad we're now back doing things our way!
Good luck