Honestly, his routine looks great.... all you can do is try to cut the nap at 1.5hrs and see if that helps. Otherswise, I am not sure what else it could be. Has he been doing this for a while?
Mine takes anywhere from 20-30mins to fall asleep and some just need to do their own winding down before they close their eyes. If its not having an effect on night sleep or naps (that is, he's not going towards the ot state), then I would let him be.
hth a little
Layla