Weight gain below 1st centile

[UnicornRainbowSky]

Hello,
My son was born mid-June weighing on the 9th centile. He had to be admitted to NICU due to meconium aspiration syndrome and was on a ventilator and sedated for 10 days and allowed home after 3 weeks in total. He was tube fed to start with and I started breastfeeding him during the day after 2 weeks. He only started breastfeeding exclusively at 3 weeks old.
He also had a tongue tie snipped whilst in hospital.
He's now doing great, back to full health as attested at the 8 week check up. He's alert, smiling, mostly content albeit the 6pm witching hour. But he's still only 3900g, sitting below the 1st centile. In between weighings he's usually putting on 18g a day on average. He's 25th centile for length. We tried supplementing with formula but he projectile vomited it all out and then refused the breast for 12 hours afterwards.
I'm due to see the paediatrician next week but was wondering if any of you have experienced similar? My gut tells me he's fine and just smaller due to his NICU stay but I do worry I'm missing something.
Thanks!

Sorry you had such a difficult time in the neonatal period. It sounds awful. Well done for breastfeeding despite that difficult start. It is a huge achievement. If he's sticking on the centile line he's on and meeting developmental milestones otherwise then he's growing well. I think the paediatrician said to me they ideally want their height, weight and head circumference within 1 centile of each other so he's perhaps still a little underweight but that makes sense given his difficult start. The other thing that strikes me is the projectile vomiting after formula. Could be he would do better with smaller more frequent feeds... but also be worth excluding cmpa as that can impact on weight gain even in breastfed babies.

Agree with Casa. Try and rule out an issue with dairy if he's vomiting after formula.

Have you read How might I increase my baby's weight gain? I found that one quite useful when my petite DD decided that she was more interested in being nosey than actually feeding

Thank you both for your answers. My diet is dairy free already so I'm not sure what that means if he has cmpa in terms of management.
Thank you @SiouxsieSiouxStiletto for the link. I already do switch feeding and breast compressions, carry him in a sling most of the day etc. I might look into getting my breast pump out again to make sure he gets the hindmilk as I do have a feeling his feeds are sometimes on the short side. And I look forward to hearing what the paediatrician says next week about my dinky chap.
Thank you again!

A couple of things on CMPA. Many babies with a cows milk intolerance also struggle with soy so could be worth cutting that from your diet too. The other thing is that if you did want to add in some formula the GP can prescribe specialist hydrolysed or amino acid formula if he is intolerant to cows milk. When we saw the paediatrician our dinky cmpa chap was 2nd centile. She said there's nothing wrong with being small! Hopefully they'll say the same to you!