Him being tall and thin, is just the way he is.
It doesn't mean its "bad."
My son is tall and thin. And he is not low weight.
The Pediatrician said he is fine and healthy as he is.
Young kids like this, also have tinier tummies than us. They get fuller, faster and with smaller portions.
They don't eat adult sized portions.
Does your Pediatrician actually say... your son is low weight?
If not then don't worry about his body shape.
Just give your son what he will eat and can eat. As long as it is not junk food.
Most young kids, eat "slow." They are not perfect with their coordination etc.
Even Kindergarten kids, eat slow.
I work at my kids' school.
And why does he only have to eat, cereal?
We eat other things for breakfast.
Its just food.
Even left overs etc.
A kid, no matter what size, should eat until they are full.
NOT according to how much is on their plate.
They need to go according to their body's cues, for hunger and fullness.
Not it being about eating EVERYTHING that is on a plate.
IF the Pediatrician actually SAID, that your son is low weight and that it is a problem... THEN, ask the Pediatrician, for food ideas.
Many people are not hungry as soon as they wake up.
And they do not eat a TON in the morning.
Again, eating and portion sizing... should be according to the child's age. Children this young, do not eat... like an adult nor the same portion sizes.
Him eating "an egg" probably makes him full, already.
If you make him eat more and more.... even if he is full and doesn't want to, then he will learn bad eating habits and he will NOT know... his own body nor his body's cues for when he is full or hungry.
Being tall and thin is not bad.
MANY kids are tall and thin.
A child's physical shape, changes ALL the time, and per age.
Boys do not have to be all bulky and stout and "big" and husky.
I know many kids, my son included, that are tall and thin.
And they are fine.
And healthy.
Don't worry about it unless your Pediatrician, said so.