Every time you change something in your turtle's tank they become stressed and unsure of their surroundings. You have to give them time to get used to their new environment, sometimes they need a few weeks. Until they feel 100% comfortable and safe, they won't bask.
A very important thing that you have to make sure is that he can get on it easily. If he fell in the water the first time then he can't pull himself up on the turtle bank. You have to try to make it more accessible to him.
Can you talk a little bit more of the yawning that you saw? Do you see it happen often? Are there any sounds coming from his mouth when he yawns? Does he have difficulty swimming or swims a little sideways? I'm asking all these questions because excessive mouth opening can be a sign of a respiratory infection.
Do you mind reading over this
http://www.redearslider.com/respiratory_behavior.html and observe your turtle to make sure that he's not exhibiting any other symptoms.