I love your choice of Maple! I have never heard of it used as a name, but I think it is lovely! Of your choices, my favorite is Denver. I think I would definitely go with a word/ meaning name. Henry, Rex and Max don't seem to fit as well. And no, I don't think that Jessica Simpson has destroyed Max for the boys... I am desperately hoping that it will prove to be an abberation. I also really like the suggestion of Wilder for you.
From your list, I like Denver. River is much too hippie for me...
Orion- the stars
Caelum- Latin for "sky"
As a mn: Equinox
Lee (a field)
Walden- a book about nature
Leo (the lion or the constellation)
Congratulations! I fondly remember your thread for Maple and how excited I was that you chose that name for her. I agree with the Berries suggesting a word/nature name for Maple's brother that isn't too cutesy or themey. From your list, I like Denver and Harbor (and also agree with a pp that Max works nicely with Maple in a different way.)
Other favorite suggestions were Wilder, Reed, Walden, Hawthorne (you could also consider the spelling Hawthorn), and Shepherd. How about Grover instead of Grove? I love Haven for a boy (similar to Harbor), though I'm afraid it's pretty girly these days. What about Hawkin or Peregrine/Peregrin?
I also thought of Rye, which I think is pretty cool on a boy, but then I thought, Maple and Rye? It sounds a little too much like a breakfast order. :)