I say go ahead. There are enough good (kid friendly) Wii U games to keep him occupied for a long time. And you are right by saying there are many great games you'll enjoy it yourself. Also, I don't know how many tv's you have but off-screen play can be really convenient if he wants to play but you are watching tv.
Also, your son won't care if NX is soon to be announced. I know kids about that age who just got a Wii, and they are loving it.
Spending warm summer days indoors
Writing frightening verse
To a buck-toothed girl in Luxembourg
NES, SNES, N64, GC, Wii, WiiU, GB, GBC, GBA, DS, 3DS, Mega Drive, Game Gear, PS1, PS2, PSP, XBOX 360, Atari Lynx