Any advice for slow weight gain baby?

Baby was born 3weeks early at 6lb, we had some early challenges with jaundice and a tongue tie that we had to get assessed/cut privately. For more than a month after the tongue tie cut he was gaining 120g per week, but he has suddenly only gained 40g in September (two weeks).

Does anyone been through the same and have any advice?

Baby is happy and meeting his milestones, smiling and giggling. I pump for an overnight feed, and usually get 4-6oz from a middle of the night pump. Baby drinks about 4.5oz from the bottle overnight. I’ve pumped during the day while on a KIT day and got just over an oz an hour, the lactation consultant said that was great because babies are more efficient than pumps and will likely get more than that.

He’s now only 10lb at 17weeks old and dropped below the 0.4th percentile at the bottom of the chart. The HV said they would review and get in touch but we have heard nothing. The GP told us we should have spoken to the HV team about some constipation, so I’m nervous about making another doctor appointment.