The child weighed 200 grams and didn't grow by cm. Maybe they measured…

The child weighed 200 grams and didn't grow by cm. Maybe they measured incorrectly last month. He weighs 7500 and 68 cm in total. My mom has been saying we can start with solids already, he needs supplemental feeding... She says she fed her own child solids at 4 months old. Now that I know he hasn't gained much weight, she says he needs solids anyway, like he isn't eating my milk. I don't know how to convince her, because she says my milk is not rich. We used to be against introducing solids before 6 months, our pediatrician said it's fine, he won't gain much more now. How old were you when