For starting cereal, it has to be 'single grain' cereal... which is easier for a baby to digest. Their digestive system is not even fully developed yet.
AND... you still have to breastfeed, ON-demand... 24/7.
For the 1st year of life, breastfeeding is a baby's PRIMARY source of nutrition... NOT solids and not other liquids.
Solids... for the 1st year, is ONLY an "introduction" to foods... not their primary source on intake.
And at this age, they do not have to, have 3 'meals' of solids per day. It is only an introduction... to solids. Having 3 "meals" a day of solids... is gradually worked up to.... over a span of months.
Your son may be constipated. Very much so.
ALWAYS breastfeed BEFORE solids... not after. Otherwise, a baby will 'wean' from breast and be too full, to nurse.
This is per our Pediatrician as well.
Do not switch around to different cereals too often... this is per our pediatrician as well. And it must be single-grain cereal.
We did not switch around cereals or other solids... until after about 1 month... of one type of solid. For example.
A "serving size" for a baby this age, is in terms of teaspoons.
I would... wait, longer... before giving him solids... or so much.
They have tiny tummies... and their digestive system is not fully developed yet.
My kids were big/long babies too... I did not give them solids until they were already 6 months old.
I exclusively breastfed, on-demand, 24/7... and more during growth-spurts... for at least the first year.
6 months is a growth-spurt period in a baby, and every 3 months is. So then he will need to nurse more and more often, they get hungrier, but that does not mean you 'replace' nursings with "solids"....
all the best,
Susan