Talk to Pediatrician on Less Weight Than Normal

Hey Dr., I need to ask my baby is 20 months old and not gaining weight he's 10.3 kg for approx for the past 7 months and looks weak too. His diet is good, and he's very active. I consulted with peads, and he said that he's all good and nothing to worry about, but why is he not gaining weight? He's also having slightly dark undereyes and some discolouration of face. I've given him some iron drops as well but it not getting satisfactory results. please suggest something