We started my baby on 120ml FM around 3 weeks too. (But to note, he’s mostly on BM with FM once or twice a day). When I visited the doctor for his one month appointment, nurse was concerned that 120ml was too much but doctor said if he’s taking it well, that’s fine. My son gained 1.1KG by his 1 month development and we thought that’s fine. He said we can follow WHO recommendation of 150ml per day for every 1KG. So if your son is about 4KG, then that’s 600ml a day, divide by about 8 feedings, that’s 75–80ml per feeding. If he needs more, then you can adjust along the way.
Every baby is different so no one can tell you if it’s too much or too little. If baby is taking it well, no frequent spit ups or vomiting/discomfort after drinking then I’ll continue feeding as per what you’re feeding now. If frequent spit ups and vomiting after drinking, reduce 10ml. Adjust from there (+5ml).