DS won’t take formula

[Firsttimemumma11]

Need some help/advice.
I’m a first time mum and my DS is now a month old exactly. He’s very dinky and now weighs 3.5 kg he was born at 3kg.
(He was born 10 days early)
Hes very slim and I would like him to gain more weight because he looks very fragile :(
I bf him and also give him a bottle of expressed milk once/twice a day.
I wanted to introduce some formula so it would fatten him a bit but every time we gave it to him he would vomit it all up! Now I’m scared if I something happens to my milk I couldn’t give him any formula.
I know he’s healthy as that’s what the midwives said.

[HappyMama01]

If he's healthy and gaining weight gradually. Don't bother.
He will grow more and more as he gets older.