I'm thinking maybe your boy likes sleeping in the rocker because he feels snuggled up?
If you swaddle him when it's time to sleep in his cot, maybe it is better?
Or is his cot warmer than the rocker? Coz in the rocker I suppose windows are open and fan is on?
Instead of only putting him in the rocker in the day time, you could also switch around like putting him on a mattress with toys around him, so there's some change in position and location...
It is possible that babies so young start developing preferences but still, it is not difficult to change as they're adaptable.
I won't advise you to introduce the yao lan as your baby will become dependant on rocking/shaking motion to fall asleep which could pose problems for you when you need to travel or go out with him and need him to sleep outside.