When to increase baby’s intake

Hi mummies, would like to ask, my baby starting to gain a little mini weight, like only few grams, and his normal milk intake seems to not be sufficient for every 3 hrs anymore. He would also sometimes cry after milk. He would already cry for milk every 2.5hrs, do i need to increase the amount ? Or just stick to feeding every 2.5hrs same amount? If increase by how many ml? Idk to follow back of tin or not. It says 3-4 months 120ml, he’s 3.5m. And fully formula fed