His weight is below the 5th percentile for his age. He may need to learn how to chew and swallow food. Some children just do not know how to do it and need to be taught. They need to learn how to chew and shallow food properly; it is not really an instinct behavior like sucking. Has he been evaluated on how to chew food? If not, then suggest getting him evaluated, by going to your MD, and start therapy so he can start to eat regular food. Suggest giving him either Pediasure or milkshakes (milk and ice cream blended) at all meals and snacks for additional calories and protein to help him gain weight. Hoped this helped you.
Thanks for the comment posted Renee *********. Yes I am giving regualar food only, but little soft, smashed with hand . He chews I guess, some food like nuts , he puts in mouth and be chewing. And the problem is he asks only for breastfeeding and he spits all the food i give. Very much frustrated seeing his behaviour, because he is so lean , to see him, he looks like 6 months baby. He walks but falls often, he does not have strength in his legs.