If it isn't the color or density it could be the shape of the hairline in relation to the rest of his hair. Sometimes it is hard to put a finger on why something looks "off" when it is as simple as the hairpiece not having the right shape to go with the existing hair.

From what you have said though it sounds more like a hair quality issue. Sometimes these things can look good out of the box but after you wear and wash them a few times it becomes evident that the hair is a little over processed. It is also possible that he is using the wrong styling product or that there is product buildup on the hair. No matter how good the product is, if you don't clarify from time to time it can make the hair look nasty.