definitely let him sleep til his normal time in the am, so that when you get there he's not already overtired and then has to "stay up 3 hours longer" or at least that would eventually be the plan. i would by all means try and go over naptime. also, go non-stop. absolutely. a layover will achieve nothing really except annoy you and him, and make you think that by the time you get back on the plane the second time you could already be at your destination.
i've flown lots with my kids, and we're making a similar trip very soon again. my dd doesn't really nap much anymore, certainly not on a plane with lots going on, but it depends on the child. does he go down easily? make sure you bring a comfort item, his lovie, whichever.
good luck!