Let's talk about naps and bedtime

My son turned 1 on the 10th. He's always been good at falling asleep on his own. I typically rock him to sleep, but we do have a schedule and bedtime routine so he knows it's bedtime. Well around 10/11 months, he decided rocking him is now a fun game. He likes to pretend to fall asleep and as soon as I lay him down, he'll stand up and play. Half the time I go back and rock him, and the other half of the time I'll let him just tire himself out until he decides he's ready to sleep and he'll put himself to sleep. I CAN'T keep rocking him to sleep. I'm pregnant so my belly is starting to grow and my son is already heavy. It's KILLING my back and belly to rock him (to the point of tears sometimes). But it takes him so long to stop playing sometimes and I just wish he'd go to bed like he used to. Any tips or pointers??