First of all, why are you making him take piano? Do you feel he'll gain something from it? Is he naturally good at it, so you want to foster that in him? Are you trying to make him learn it, because that's what you feel society expects? Are you doing it because it's something you enjoy and want to pass that on to him? Additionally, why are you being his teacher?
It seems to me that he doesn't like playing piano, or at the very least, he doesn't like you giving him lessons.
If he genuinely likes playing piano.
If he likes playing piano, but doesn't like you teaching him (ie - he'll play it on his own, or showed an interest in it previously), then hire a teacher. The student-teacher dynamic is different from a parent-child or spousal relationship, and is and can be often more demanding than the latter two (for example, my basketball coach could yell at me and it wouldn't bother me, but if my parents were to do that, I'd react very differently). If you play piano, and especially if you're good at it, he may also be intimidated by you, and see your skill as big shoes he's expected to fill. If so, then his behavior is his way of channelling and communicating that fear.
He may be more motivated simply because it's not his mom teaching him.
Additionally, he's starting to get to that age where mom and dad aren't the center of his world, and getting into the age where he wants to assert his independence. He may be feeling like you're smothering him by being his piano teacher. Having someone else take on that role may give him the sense of freedom he's looking for.
Another way you can help motivate him is to find shows or competitions that he could participate in. Let him pick the music and instill in him the importance of practicing so that he does a good job. This will give him a sense of purpose to the learning, and a goal to work for, instead of just aimlessly learning a skill.
You can foster his talents in other ways, without being his teacher. Buy him a keyboard, both for the portability, and for the variety. Get him music books with music from his favorite movies, shows, or artists (if you can't afford to hire a teacher, this is probably the next best thing, because it gives him something he can relate to and be proud of; very few 8 year olds can really understand/appreciate the classical greats, and even if your son can, his friends probably won't). Maybe see if you can arrange a meeting with his favorite local musician (or a local musician in general).
If he doesn't like playing piano.
If he doesn't like playing piano, then it would be a good idea to consider your motivations for trying to force him to learn it. Forcing a child to learn piano when they have no interest is, in my opinion, no different than forcing a child to play sports when they have no interest - it's often done because it's what's expected ("boys don't dance/play piano/cheerlead, they play {insert sport here}"), and not because that's what the child wants to do. Forcing your child to do something like that solely because it's what you believe they "ought" to be doing will do little more than foster a disdain, or even downright loathing, of both you and the activity in question.
What does he actually like to do? What does he want to try? If you want to foster his talents, then foster his talents, and what he's interested in, not what you think he "should" be doing. If he's interested in something, it will be significantly easier to motivate him (and if you still have trouble motivating him for the "daily grind" aspect of his chosen activity, most of the above "if he likes it" tips can be modified to suit).
Learning piano teaches discipline, patience, and a respect for art, yes, but so can learning a different instrument, or pottery, or painting, or theater. Things like sports and martial arts can't teach respect for "the arts," but they can teach discipline, respect for others, and respect for their fields, among other valuable lessons.