what time does he get up in the morning?
You could try dropping the nap, but offering some quiet activity instead. Then try for in bed by 12 hours after wakeup. (or better yet, try for asleep by then).
My DD just dropped her nap, and it moved her fall asleep time from about 9pm to 6:30/7pm.
Watch for overtiredness, though, and be prepared to offer the nap if it seems needed (ie on any given day, just not every day).