Waking a baby for Night feeding...

LO is 6 weeks. He usually is good about waking for a feeding, but I occasionally have a hard time getting him to eat at the 3-4 hr mark. Any suggestions on how to get him to wake enough to feed?

He was up because he 💩 and I changed his diaper, then immediately began to nurse, then he fell asleep 2 minutes later and I couldn't rouse him. I know he didn't get much breastmilk since I expressed before his diaper change (I have to express beforehand otherwise he gags/chokes from the flow).

I will mention he is pretty tired from 💩. He strains a lot, because he is going through infant dyschezia and it wears him out. The only thing I can think of is just wait another 20-30 minutes to wake him...