Talk to Pediatrician on My Son Is 19months Old But Is Not Gaining Weight As A Normal Child Gain..

Hey! my son I'd not gaining weight, he is 19 months old and weight around 8.7kg, he is active MashaAllah...completes his all milestones but I'm worried of his weight...why he is under weight when his appetite is good now he is on whole milk in which i add 1sp of pediasure before that he was on top feed for 16months but he also have a good intake of solids etc...I'm worried about the weight alot...