I think I like Lennox. The first syllable is soft and smooth, but the x on the end gives it some punch and makes it interesting. It makes me think of Lennox Lewis and Lenox china which is a cool dichotomy, actually...brawly meets refined.
It's definitely usable in the US, and, since Lennon's overwhelming celebrity connection and trendy two-syllable-ends-in-N sound don't impress me, I prefer Lennox. I wouldn't pre-select a nickname, but it wouldn't bother me if Len or Lenny seemed to suit him and ended up being used. I don't like Nox at all.
My favorite of your combos is Lennox Peter.