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Your child is prone for fluid aspiration in lungs and chest infections because of weak strength of chest movement. He needs physiotherapy
to increase strength. Never led him drink fluid in lying down position. With physiotherapy and speech and audio therapy he will hopefully be able to be somewhat sufficient by may require care of elders for life because his brain function will never return. He needs support of others to live. To gain weight he can be given high protein diet like pulses, dals, eggs, chicken, lean meat. Infections can also cause loss of weight. Follow up with Paediatrician for his outcome and prognosis as well. GOOD LUCK !!!!