Hi candy,
It could be due to his stomach hving difficulty digesting food and he may not feel good. Is his stomach bloated? TCM can help his condition. My 16.5 mth old son also look skinny, as his stomach is not good. Now he is seeking treatment at Yu Guo, but TCM takes a long time to take effect so hv to be patient. Most importantly, yr son's digestion must be good and it'll absorb all the nutrients from his food.
What food are you giving him? It's better to give him soft porridge as it's easier to digest. You can cook si shen powder in his porridge, this helps to improve his appetite. You hv to be careful in the food you give, don't give food that are difficult to digest as it can weaken his stomach in the long run.
Maybe you would like to share what his diet is like. Don't give too much multivitamin at a young age, as it'll add a load on the liver cos vitamin supplements are not natural and contain chemical additivies. It's better to obtain nutrients from food.