Fable and Amity are both word names, and I would like to see that theme continue with your son.
I would strike Wolf, Wiley, and Zen from the list. Wolf feels too aggressive with Fable and Amity, and wily doesn't exactly have the same positive meaning as fable or amity. I would strike Zen mostly because I dislike the "borrowed from Buddhism" names; they come off as hokey attempts to be hip and New Age-y.
My favorite choice for a brother to Fable and Amity is Harbor. I really love the way the dictionary definitions of these words tie them together as names. A fable is a story with a moral, just as every life is. Amity means friendship and peaceful harmony, two things we all need in our lives. A harbor is a place of shelter and refuge, which is, again, something we all need at some point in our lives. Friendships can give us places of refuge, and that in itself creates a story (with a moral). So yes, I really love Fable, Amity, and Harbor as siblings.
What about Harbor Wolfgang, Harbor Lelan, Harbor Truett, or Harbor Theron?
I like Lelan! I also like Zephyr.
Here are my suggestions, keeping in mind we have completely different tastes. :)
Wolf/Wolfgang - DH isnt sure about this one though - I dont really like it, Wolf is cool but no Wolfgang.
Valor - Valor would suit your Girls so much!
Zephyr - Dont like at all.
Truett - Dont like at all, again.
Wiley - It springs to mind Wiley Coyote, but i guess its a neat name!
Cypress - My sisters best friend is named Cypress and its a she... so not for a boy i'm afraid.
Lelan - A family surname. Could potentially make a neat middle name!
Zen - Possibly Zenith or Zenon. Maybe just Zen! Dont like it, sorry.
Roan - Roan is a horse coat pattern, and growing up around horses i can only associate it with them, not a human baby, sorry.
Theron - In my native country, this is a popular last name. I can never see it for a first name.
Harbor - Its okay.
Claude - I think Clyde sounds a little better.
River - I know too many girls called River.
Here are some i suggest:
Good Luck! :)
From your list I really really love Valor and River to go with Amity and Fable!
I like the name Theon better than Theron, personally, but I think either one could well. And instead of Wolfgang to get the nn Wolf, I like the name Lowell which means young wolf.
Andddd, somebody suggested Griffin and Phoenix which I think are two great names to add to your list!
Good Luck :)