5 month old baby boy not enough drinking milk. I'm at my wits end. Please help

I'm hoping someone can share their experiences if they have been through the same thing with their lo. My 23 week old baby boy has never been a good drinker ( formula fed Hipp combiotic) from birth. He was 2.9kg at birth and now at 23 weeks only 6.7kg. He drinks between 600-720ml milk a day now. We had a good week ( last week) when he drank 800-900ml ( prob a growth spurt) and during that time, I also introduced single grain oats with some peanut butter to him. He loved it and cried when he finished the portion cos he wanted more. After that great week of drinking, he is now back to taking only 70ml-120ml per feed. He drinks 6 times a day. Last 3 days he has been drinking 600-620ml ( total per day).

Besides his poor drinking and being on the lighter side, he has 5-6 wet diapers a day, poops at least once ( yellow mustard) , is active and happy when he's awake and hitting every milestone. He also sleeps through the night since he was 4 months old ( I sleep train him). I am at my wits end with feeding him milk because I have tried every trick in the book to get him to drink more... And it never works. I tried dream feeding before we sleep train him and all he did was stopped drinking in the day and drank more at night. In the end there was no difference.

He also has a tendency to projectile vomit which makes me really stress. I don't want to keep forcing the teat into his mouth as he cries and kicks really hard when I do that. Also I am worried about creating milk/ bottle aversion. Yesterday after his first nap of the day, he only drank 90 ml.. so I put him in his baby highchair and gave him oats and peanut butter.. and this cheeky boy ate everything. I'm guessing he just doesn't really like milk... But he's still too young to go on solids only. I'm planning to start baby led weaning when he shows interest and only gave him purees now cos the pediatrician told me to.

On another note the ped I saw ( I'm changing to another one) said he needs to put on more weight because he only has one chance to grow. I immediately refused to believe that my son only had one chance to grow. He still has toddlerhood, puberty etc. what an idiotic thing to say cos that statement really disturbed my husband.

Anyway did anyone go through something similar with their 5 month old not drinking enough milk and what did you do? Thank you so much in advance. Xx