I'm 5'10" 175lb and I would get my head caved in on my first shift. I have no idea how he plays at that weight.
I don't think he has much choice.
This was always the biggest knock on Pettersson right back to the draft. He's tall but he's built like a beanpole and quite frankly...doesn't seem to have the type of frame that you can really "build up" to much more than that. It impacts his skating too. Where it seems like the power in his (inefficient) stride just dwindles over time as well.
Listed weights don't really mean much of anything. I'd imagine he probably started the season at something a ways heavier than that. But if he's got the sort of build that just can't build and keep mass, it's always just going to deteriorate over the course of a long grinding season. That's the case for most players really. But if you're starting from a compromised position, it's only going to get worse from there.
Until he's just getting outmuscled on every puck battle. Falling over. Lacking any semblance of explosiveness in his stride. etc. Which...combined with what seem to be some mental focus/intensity sort of hurdles, just makes for a lot of frustration.
Not sure what you do about it. But he's gotta figure something out. He simply has not been good enough.