I recommend you to see the Doctor and if he says ur baby is fine just follow his advice, there is nothing wrong with his weight, and probably that's his body type as long he is eating and is healthy I wouldn't worry. My advice don't try to over feed him and maybe just ask Doctor if there is a need for vitamins. At last do not let people comments to affect ur mind and to make you worry (babies don't need to look fat to be healthy)trust in God and enjoy your baby.