dont be too hard on yourself.
some kids have this habit of cryin then fall aslp.
my dad say i was like that too. he said when i want to sleep, i cry, when i wake up i also cry.. ahaha~ take it easy. do you prepare him for nap time? or you let him sleep on demand?
if he sleep on demand, he might be already overstimulated, thus get cranky n cry.
if possible, give him a routined n fixed schedule to nap n sleep.
prepare him for his nap about 15-20 mins before actual nap time.
as in, bring him to the room, off the lights/draw the curtains, position him in sleeping position, maybe sing him a lullaby/pat him gently. tell him its time to sleep n slowly let him doze off. or also, can give him milk to sleep.