The intensity of these episodes varies and for your son, it may occur when he is about to be put down to sleep because this is a time of separation from you. You certainly seem ahead of the game since you have already found successful ways of encouraging him to relax and to get ready to go down for a nap. I would continue these techniques or any others that seem to relax him.
When putting your son down for a nap, give him an extra hug and kiss and say "It's nap time, I'll see you soon" and exit the room quickly. Keep the door of his room open so that he can hear you in the next room. If he cries and calls after he is put in the crib, you may enter the room and say something like, "I'm here, but it's time to sleep now" and then leave the room quickly again. Try to avoid the temptation of picking him up -- this will only reinforce the behavior you want to change. You may have to do this more than once until he falls asleep.
Overall, all children go through a time when bedtime becomes a battle. Continue your calming bedtime routines and reassure him that you are nearby, but that it is time to sleep.