Constipation caused by dry air?

My 6mo son has started this routine of pooping only every 3 to 4 days, which his Dr said is normal. Our concern is that he's also started grunting and groaning alot while scrunching his face like he's is discomfort.

My wife took him to the in-laws for a week and while there, he pooped daily, but as soon as he came back home, he was back to his 3-4 day explosive schedule.

We've been struggling with dry air, using 1 to 2 gallons in our humidifiers, daily, just to keep us at 50%-60% at night while sleeping. He still pees a lot and farts all day and so I don't think it's dehydration.