HELP! Weight and Developmental

My currently baby boy who is 11 month plus still not gaining weight.. He weight now is 7kg.. Before he warded in hospital due to Acute Broncholitis, he was chubbier side.. Since he has been warded 3 times for it, he doesn't gain weight much.. milk intake also lesser, he havent start BLW.. He only took puree, porridge and oats cereal.. But that too he never eat alot.. Im worried cause his development skills all delay.. he haven't start to sit on his own, have not yet stand or walk.. Crawl too he just drag his body, he never lifted up his stomach to crawl.. I just worried for him.. He too has fallen bed 6 times on hard wooden floor and tiles and hit his head 6 times.. #advicepls #pleasehelp