Thank you for the input.
3 weeks early deliver is usual.
It does not seem to be cerebral palsy.
Small baby can stretch normally.
Some may do a bit more often.
As he is doing it mostly after feeding, it is likely due to infantile colic or reflux as you thought of.
The cause of colic is not clear.
You can hold him upright after each feed.
Burping can help to expel gases.
Reflux can occur in small babies too.
Sometimes medicines can be required or formula may need to be changed.
During next checkup, doctor can prescribe medicine if it is needed.
I hope this information will be helpful.
If you have any query, please feel free to ask.
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Wish his healthy life.