Baby not eating anything

Hi all,
My almost 25 months old baby boy(12kg weight, 81cm tall) is having some eating issues off late.

He had a cold two weeks back, ever since he recovered, he is a very choosy eater. He only takes fish and egg whites, nothing else, no bread or rice or any other carbohydrates, no cereal or pulses, no fruits or veggies, just his formula milk& egg& fish.

His poo is smelling bad, it's somewhat solid and little blackish ( it used to be creamy, less smelly and yellowish before). He scratches his anal area, but no bumps or discoloration. We had dewormed him a month back.

Pls suggest something. I really realy want my baby to eat healthy...he is becoming lean day by day...

He isn't sick, is very active, plays n runs around, no other noticeable change in behaviour or health status.

Thanks for your time, awaiting sone solutions

Thanks