It seems he is getting insufficient amount of milk .. Y not try adding dairy to his food ? U may wanna try offer him yoghurt with fruits and chia seeds . N introduce cheese to him . Cut down the amount of porridge and make sure u let him have his milk first before food . That way, he won't eat too much , and he would rely more on his milk . Later when he is 1 years old , u may give him more food than milk .
I think baby below 1 should have milk as their main food. maybe you want to cut down on solid and make food using milk
https://sg.theasianparent.com/the-many-faces-of-picky-eaters-part-4