With no answer yet on with or not he lied, my response is assuming he didn't. Otherwise I'm with BigDawg.
I would make sure I saw the grade on that homework. If your son typically gets B's on his homework and on this one he got a D.... two days no wii. If his completed the work and received an acceptable grade, I wouldn't have an issue. Maybe he knew the level of effort and time it would take and whil cutting it close still managed to get it done.
I have two sons in college and one in high school who still do that... but they get good grade managing their own schedule... so I bite my tongue.
On a similar note, when they were younger they were not allowed to have music on while doing homework. In jr. high they negotiated new rules to allow music as long as they kept their grades up. Turns out, they didn't inherit my difficulty studying in noise environments. Show-offs!