Sounds like you have a lovely relaxing routine for him which he enjoys. I would wonder if he might need a bit less intervention/help from you at this age, the contact even in this peaceful way is stimulating for him. So perhaps if you do what you're currently doing but wind it down towards silence and stillness and see if that helps him. Slow the back rub down, slower, slower, slower, and then just stop until you're just resting your hand on his back, and the same for the shushing, just shush slower, more quietly and stop. Stop if you can before you judge he's at the very drowsy moment.