K.S.
I also have worked in a daycare and am in the process of starting a home daycare.
My experience with infants 8 months-15 months has taught me that sometimes you can leave them and they'll pass out on their own. Then in cases like yours you can't, I would suggest laying him down on his side or tummy and rubbing/patting his back. Playing soothing music/humming/singing also helps. It might take a couple weeks for him to get used to the idea, but he will learn that once you lay him down, cover him up and start patting his back that he's not getting up and he will go to sleep.
He clearly still needs naps or he wouldn't let you rock him to sleep, so just stick with "helping" him go to sleep in his crib/playpen. In time it'll take less time to pat him to sleep and he might even relearn to layhimself down and go to sleep on his own.
Hope this helps,
K.