Concerned about 7 week old baby at 0.4th weight centile

My baby is 7 weeks old and was born just a few days shy of term – he now weighs 3.6kg. He has not grown as much as he should have, and in those 7 weeks his weight has decreased from the 10th centile to 0.4th centile, length from 50th to 15th and head circumference from 25th to 9th.

He is mostly breastfed, supplemented with formula which we have increased from 1 to 2/3 bottles per day as of around 10 days ago. He has always seemed completely normal in terms of feeding and has never shown signs of discontentment following a feed. He rarely spits up milk and his nappies are typical in terms of frequency. His very early development seems fine too as, for example, he can follow us around the room with his eyes, smile and lift his head up for a while.

My partner and I are not small people (6’2 & 5’5), so that is not the reason.

Health visitors are monitoring but haven't seemed greatly concerned. Quite frankly though, they haven’t been very helpful throughout.

He has been gaining around 40g per day on average since we increased the amount of formula which is encouraging. However, it still seems strange that he is so extremely small and these gains only keep him from dropping below the 0.4th centile weight trajectory rather than making significant movement to the trajectories above.

Has anyone else been in a similar situation? If so, I would love to hear your thoughts. Thanks.