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?
We had this with my first. Low weight gain for several months. Not ever darkie to thrive, but always low percentiles and fell percentiles in the first month. Our second lost 11% of her birth weight initially (and we had a gazillion appointments) but she's now thriving. We tried lots of things (tongue tie release, supplementing etc) so feel free to ping me for commiserating or questions. I will say our eldest who consistently has low weight gain is 2.5 and a healthy, happy toddler. He's just in the 15th %.
As long as your baby is at least 1 month old you can add 1tsp infant oat cereal to 1oz of liquid formula (use a bigger flow nipple) it adds extra calories and helps with reflux as well.
Or you can add it to pumped breast milk as well same ratio!
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.