If he had a hair transplant back in 2003 on his receding corners we shouldn’t be able to see them receded in 2015.
Although further hair loss on his mid-scalp would be acceptable and could be explained as progressive hair loss with his age, the corners should look much more denser (assuming he has done a hair transplant in 2003).
Way more likely than a hair transplant, the photos and video footage from 2015 suggest that he is in fact wearing a (partial) hair system, which we can see in all of his movies, interviews, TV appearances and red carpet events.
When his hair is buzzed, it’s obvious that the frontal hairline is lacking the density (especially on the corners).
The diffuse thinning on his mid-scalp can rarely be seen in his movies. It’s simply because he is covering it with a partial hair system/wig.
It’s not 100% sure whether he is using a partial wig or a full head wig for the entire top of his scalp. It’s likely that he has both types of wigs.