4 month old freak out.

Jessa • Unofficial nomad. Aspiring midwife. Fierce lover of naps.
The 4 month old I care for has started screaming, no, SHRIEKING like a banshee being sawed in half when it's nap time. Here's how it goes:
He has a 6oz bottle, I burp him between every 2oz so his gas isn't too bad AND I give him gas drops (he's really struggled with gas). After the bottle I try to burp him again and let him sit up for 10 minutes or so before I lay him down. When I lay him down, chaos insues. He cannot keep it together! But as soon as I pick him up (after 5 minutes of screaming) he falls asleep in my arms, but when I lay him down again, he freaks.
Since he's able to calm down when I pick him up, I'm assuming he's not in pain- right? Any idea what could be wrong? Any idea what I can do to help him?
I don't see his mom much, so we don't talk about his nap schedule or screaming habits, but I will bring it up to her if this continues.