Sorry you are having a rough time.
Sine he goes to sleep fine at daycare by himself, we know that he knows how to go to sleep on his own and he can do it!
I have a little guy at my home daycare who just turned two--he naps well for me, but sleeps with mom and dad at home and does not nap regularly for them. I have had others as well who nap at my house but not at home. In my experience, with the toddlers, it is all about expectation and consistency. At my house, they have to lay down for a nap. While I am nap training (which I had to do for all) I did all of the BW techniques, but I was consistent. It took a long while, but the kids learned that naptime was naptime. This didn't always carry over to home because the same amount of effort wasn't put into it.
Have you tried PU/PD? You could do that, or I do a adapted version of this at this age.
With a 21 month old, I would put him in his bed and then stay in the room. Anyone can do this, so get help if you are about to have a baby! See what works for your guy, but I either have a hand on their back or I just sit by the crib. If I am sitting by the crib, I bring something to do--like a book or I play on the computer. I'm sure he will throw a nice fit, but eventually he'll get the idea that it is naptime and go to sleep. It depends on the kid of how long I stay in the room--with some I was able to walk out when they stopped screaming and laid down. With others I had to stay until they fell asleep for a few weeks. It never took longer than 15-20 minutes though.