Sleep begets sleep and I would bet my last dollar he is so over tired that the neurons in his brain simply can not settle long enough for him to get any sort of quality length of sleep.
If he were here I would put him down to sleep the second he gets here. 8am, tops. I would have him sleep at least an hour, and yes, you might have to allow some extra time for him to CIO until he actually goes to sleep and sleeps for an hour. So, if that means as soon as he gets there then so be it. And then nap him again in the afternoon.
He sounds like the classic over-tired kid. If you get him sleeping MORE and on a more frequent basis I will bet that before long he will be an awesome napper.