Underweight son

Ja

Anyone 1 year old under his weight chart? He’s in 6th percentile for height and 15th for weight. Only 29.5inch and 20lbs 5oz and they are concerned. They put him on a fatty diet for a month and he still hasn’t gained weight. He doesn’t drink cows milk because he literally hates it I tried different milk he hates it unless it’s breast milk. I’m curious and a little desperate at this point. Any help would be great, thank you.