Suggest to up to 3 meals a day at this age. I brought my bb to the PD at nuh coz I was also worried about his stagnant weight gain, and the pd said tt the baby needs to eat more solids at that age alrdy coz the milk doesn't provide enough calories. After I did tt, my bb did managed to gain some weight, though affected by him falling sick in between.
Can also try feeding your bb with more calorie dense foods like egg yolks ,nut butters if not allergic