YES!!! Pat/shush can certainly become a prop.
This happened with my little one and I had to stop using it and move to pick up/ put down.
If you are continuing to use pat/shush, try and make sure you are slowing the pat down as you see the eyes getting heavy and then stopping it entirely BEFORE they fall asleep. Your aim is to have your DS fall asleep without any props.
My DS is now 1 year old but uses a "bunny" as a comforter to suck on before he falls asleep. But because of his age, if he wakes up in the middle of the night and needs it, he just grabs around for it and sticks it back in his mouth and goes back to sleep without bothering me.
You should expect your DS to be able to get past that 40 - 45 mins without your help once he is able to fall asleep in the first instance without any help. This didn't happen with my DS until he was about 4 - 5 months old.
So keep perservering!!!