It sounds like a number of different things might be going on.
My oldest is an avid reader, as well (he's in 6th grade now,) but his reading goes in fits and spurts and is definitely affected by the rhythm of the school year. So, for example, he devours books over the summer and at every break from school (he read four book series just over the two-week Christmas holidays, for example), but then will go weeks, if not months, without doing pleasure reading while school is in full swing.
I've asked him before why this is, and his answer was that during school, he likes to focus his free time on his friends (chatting on his laptop) and on simply unwinding (listening to music on his iPod or playing computer games.) During the school year, he is also a competitive fencer, and goes to practice 4x/week, which I know is physically tiring. And although school isn't terribly challenging for him, the daily grind of the school schedule is tiring, so I think he doesn't necessarily want to expend the extra mental energy on pleasure reading (no matter how much he loves it.)
Another aspect to consider is that your DS might be putting forth more effort this year into building his social skills and fitting in with his peer group, whose interests clearly revolve much more around TV shows and music than around the latest book they've read, especially since your DS's reading level is so far above theirs. Watching him spending his free time immersed in popular culture instead of a pursuit you find intellectually appropriate can be frustrating, but really, it is so very normal, and, in fact, quite encouraging, as it speaks to his interest and ability to make friends and become socially competent. Don't underestimate this skill set just because it doesn't seem "academic." Dealing well with others will provide your DS a huge advantage throughout his life.
Finally, another factor to think about might be his choice of reading material. I'm sure plenty of the other parents here can weigh in on the intrinsic difficulties that come along with choosing books for a child who is reading at a level far beyond their chronological age. Finding the sweet spot between reading material that keeps them engaged while not dealing with subject matter that is not yet appropriate is hard! Maybe if you posted what he's already read/is interested in, we can help you with some reading suggestions.
Other suggestions that come to mind that we do around here are the following:
1. DH and DS have a weekly chess match; it's become their "thing" and they bond over it, and I like that it's more mentally stimulating than watching football on TV (which they also do together, arguing over calls and strategy, lol.) The point I'm making here is that it's a time where DS is engaging with one of his parents, doing something they both enjoy, AND it requires some mental energy at the same time. Win-win.
2. I read aloud to DS, often books that are more challenging (although not beyond his ability to read himself) AND that have more advanced subject matter, as well. BUT...I'm reading them aloud to him, so we stop and discuss the issues that arise, rather than having him read them on his own, without any additional input. This is also a win-win, because it doesn't require as much energy for him to listen to a book, so we do this during the school year, we're getting to spend time together, AND he is still challenged and engaged in reading. We do this a few times a week, as time and energy permit.
3. Family game nights: okay, so these are not always super intellectual (especially when forced to play Pretty Pretty Princess with his 6 yo sister, lol), but we usually first play a game with the littles on their level, and then after they go to bed, we play Scrabble or Settlers of Catan or Risk or the like, which is much more fun. Sometimes we do logic puzzles and brainteasers as a family, just because. We'll do them at the dinner table or while I'm prepping.
I think the common thread behind all my suggestions is that they're aimed at fun first, with learning as a side benefit. All of them strengthen our family bond, and are fun ways to spend free time, which is why they work!