oh hun, that must be so hard!!!
can u try get him to have a shorter sleep earlier in the day?
Does he have any nap time triggers? like a glass of milk or anything?
DD1 still goes down for an hour, she doesnt sleep but she has some quiet play time in her room, if she doesnt she's feral by tea time.
She knows that once a certian cartoon is over its time for a rest.
Sorry not much help really, hope u get something sorted