Under nutrition could be your problem.In some case when the birth weight is low and subsequently the child fails to gain weight.Low self defense can lead to recurrent infection and with each episode child loses more weight so this vicious cycle.Try to give him nutritious foods like fruits,vegetables,and calorie rich diet.feed him in short duration throughout the day.Get him examined by pediatrician for possible cause of his weakness.For speech after consulting your pediatrician you can take help of speech therapist if rest of the things are normal.