my baby boy completed 9 months today and his weight was 9kgs and birth weight was 4 kgs. it's ok

A. Your child should have been 8 KG by six months and nine kgs by the time of eight months so right now this is okay it can of course be better if only I go by birth but currently with what you have mentioned is decent enough try and increase the weight over the period with a good and healthy diet that you offer