I wonder if he would do better with you if he was there all day? For the reasons the others mentioned. Is mom giving him a morning nap? That may throw it all off right there.
At this point, there is nothing more you can do to "help" him fall asleep. I keep it very consistent for sleep here. We have a bum change before lunch, I clean them up after lunch, then take them one by one from the highchair to their nap zone. Sing a little song, have a QUICK cuddle while singing and then lay them down. Walk away, close the door, and come back at the end of nap/quiet time.
It gets so that the little ones are saying "night night" to the other kids as I am unbuckling them, LoL.
It is super hard to hear them cry, but I REALLY strongly believe that it is in the best interests of everyone. Mom has to be on board here, or it is going to be even harder, on HER SON.

And it does sound like the other little one is just a little stressed out by this. But have mom and dad mentioned anything? Maybe some changes going on at home?