Been dealing with the same thing. Started throwing in a breast milk bottle an waking him up more cause he would sleep so long if i didnt. An his been gaining. Its not super crazy but its good
My daughter barely ate and only gain like an oz per month. (3 months to 6 months and there was no health issues, she started eating better when we introduced solids)
Are you breastfeeding?
The first few days I was having this problem but then she started to gain weight. Most doctors said it’s normal as long as they are having enough wet/dirty diapers.