Spiderlashes, I'm sure that he's tired because he yawns and his eyes get heavy. Normally, when he's tired and I put him in his bouncy seat, he won't fall asleep. I hadn't tried any of the methods you mentioned until today. NvmbrCurlss, I thought about that! According to his mom, he doesn't get a lot of sleep at night and hardly naps, so it's very likely that he's overtired once nap time comes. I've created a routine for him.
Now for the great news! Today, I laid him in his bed right when he yawned/rubbed his eyes, and he was in a dimly lit room (thanks, Spiderlashes!!!) I hummed a tune, and gently patted him, and he fell asleep! He slept until his mom arrived (50 minutes) and she woke him, but he could have slept longer. I plan on following this same routine consistently, while slowly taking away the humming and patting, and pretty soon he should be falling asleep on his own. I'm going to tell his mom about the new routine! Thanks for the help!!!
Another question: Can babies get stressed? My cousin's parents don't have a stable relationship and they fight and argue a lot. I'm wondering if he can sense their emotions?