When we started homeschooling/unschooling, DS only wanted to play videogames. Finally, I was frustrated over the lack of interest in anything but Pokemon.
M: So, just what do you think you're learning on those videogames?
A: Self-confidence mom - I'm really getting good at this and learning self-confidence.
Could it be television is your child's way of decompressing after 9 months of school? I know 8-10 hours a day is definitely not recommended by all the child experts, but it might be his retreat, his need to regroup.
You mention he plays legos while watching - television just might be background noise for him. My son actually thinks and focuses better with music playing (and, yes, we have Pokemon music).
Electronic entertainment does affect my son's behavior, so we need to take brain breaks where we go for a walk - walk to the library and rent videos if nothing else - go swimming, ride a bike, pick up legos - whatever.
I explained to him that he seems happier when he has his brain breaks and television shows have an awful lot of food commercials which make him think he's hungry (he knows he has a bit of a weight problem). We agreed to try it as long as he gets to go back to his activity of choice after a brain break. He's not losing any weight, but getting up and walking, riding, swimming, does break his focus so when he's done he switches to gaming instead of television. And, he's a much nicer person when he gets his breaks.