My 6 week old seems to struggle so much to poop now and it still comes out watery (exclusivelt breast fed). What's up? How can I help him?

Avoid cow's milk . Many infants have an difficulty digesting cow's milk protein that comes through breast milk. You need to follow a strict dairy free diet and see if his stools improve over the course of 5 to 7 days.
Infantile dyschezia. Infants are still learning how to deal with the feeling to poop. Instead of relaxing they end up contracting that sphincter. That is the reason they make all those faces .If you think about it its pretty hard to poop laying down. If it is coming out soft and runny you dont have to worry .
Likely gas/will pass. It is easy to attribute feelings that we experience when we see an infant struggle. The fact is that they are wired to do similar responses for many issues. When tired or fussy, their upper arms fold at the elbows & come in to the body, the knees come up and the belly pooches out. Whether this is gas, cramps, tired or mad; they appear the same.This subsides some by 4-5m when they can roll over.