He can't release his gas.
Gas can be released from burping or farting.
My daughter as an infant, RARELY burped or farted... even though I burped her.
His behavior: writhing/grunting/body tensing/ looking uncomfortable are classic signs of gas, and a baby acts like that because he has gas, but it can't come out.
What we used for my daughter is Mylicon Infant Gas drops. It works real well. You should get some. Any drugstore has it.
He is only 2 weeks old... so their system/digestive system is still very immature and not even fully developed yet. They don't automatically know how to burp etc.
I doubt it is reflux.
And yes, gas causes pain and discomfort. That is why he acts like that. Their body does tense up.
After feedings, don't lay him down right away. He just got his tank filled up. If laid down right away... then it can up-flow. Not necessarily meaning reflux. He is full after a feeding...
And yes, many babies spit up. Which is normal as well. My son was a big spitter-upper. He did not have reflux.
Your baby has gas and it is not coming out...
all the best,
Susan