4 month old needs to gain weight

MW

My baby boy just had his 4 month checkup yesterday and the doctor is concerned with his weight he went from the 30th percentile to the 3rd. He is fed exclusively breastmilk by breastfeeding and pumping and bottles.

She advises to start adding a bit of cereal into his diet for added calories it I'm wondering if anyone else has experienced this and has additional suggestions