It may be because the filter is too strong, or the fish is ill, or both. The current of the tank, along with the pull of the intake, may mean the area near the intake is where he can stay while expending the least amount of energy. If he can go into a cave or decoration, he would also get relief from the current, but sometimes it takes a fish a while to figure this out.
Keep in mind that fish do all kinds of things for all kinds of reasons, and this is only one possibility. It's also possible that nothing is wrong and he just decided that he likes that area for reasons we'll never understand. It's important not to make significant changes on an assumption. When you notice an issue like this, you can make small changes until the problem is solved.