18 month old not napping

I have had this 18 month old for 6 months now. For the first week he would nap solidly for an hour in the morning (then I would wake him); then absolutely no nap in the afternoon. I then took away the AM. nap...since I have never gotten him to successfully nap...only once, when he was sick with a head cold did he sleep for the last 15 min. of nap time (of course, had to wake him). I have a monitor in his nap room and can hear him chatting the ENTIRE 2 hours of nap time, every single day. In the period leading up to nap time he is yawning, visibly tired. He happy toddles up to his nap room with the rest of the group. I've gradually bumped up nap time to now 11:30 thinking it's too late? I speak with him every day putting him in to his pack n play, "it is nap time, time to sleep". He answers "ok". Lately, he will throw all of his blankets, pants, socks etc. out of the pack n play...at which point I go in sternly lay him down and repeat "nap time"...about 20 min. of loud screeching ensues. He naps perfectly fine @ home for mom & dad and is otherwise a great child to have around.

I should also add that I have duplicated everything they have at home...white noise machine, darkened room, his blanket.

Does he sleep on his own or in a shared room?

On his own (here and at home)

Does he actually sleep at home or does he play in his crib too but parents think he is sleeping because he isn't crying to get up.

Also do they use a packnplay for play as in he is conditioned to sleep in his crib but to play quietly and entertain himself in the playpen while the parents are busy.