starting solids

my baby was born on 26 sep so he is 4.5 m old, his weight is on the lower side and he is formula fed but is not very interested in milk. per day he drinks at max 6 x120ml sometimes more. so should i start giving him rice cereal? Doc is saying to wait till 6 months but i want to know if somebody has started solids at 4 m+?

PS: i want to know personal experience. i will do what’s best for him 🙃