When did u give your baby solids baby food?

mY BABY IS ALMOST FOUR MONTHS he is always hungry even right after I feed him and today I put a spoon in front of him with some gerber sweet potatos and he lunged for it before I could even second guess myself he had eaten it my doc said to wait until six months but the poor thing acts like he is starving all the time I have started him on rice cereal in bottles doesn,t seem to be doing much for him