Perhaps he'd like Cara, Clara, Mara, Lara, Vera, Farrah - all rhyme-ish with Sarah.
Elisabeth is gorgeous, but pretty standard as a middle name, although the 's' spelling makes it prettier and slightly more stand-out - though I do say slightly.
Maybe Elisabeth as a first name would be more suited to Atticus? Or maybe Elise, Eliza etc...?
Btw, I love a pp's suggestion of Sadie! Atticus and Sadie are lovely together.
I actually like Sarah Elisabeth! Yes, it's pretty traditional, but it's almost refreshing in this day and age where everyone gives their kids creative/unique/unusual names. Even though there were probably 9 Sarahs in your class when you were in school, I hardly hear it these days (I'm a substitute teacher). She would probably be the only Sarah in her class. I also think Sarah and Atticus work as a sibset. They're both vintage-y. I love the suggestion of Sarah nn Sadie with Atticus. You could also pair Sarah with a less-traditional version of Elisabeth, like Elspeth, Elise, Bess, Lisette, Elisheva, Elisabetta. There are so many!
If you wanted to go for something that still felt classic...but not as common as Sarah (especially the combo Sarah Elisabeth...)
Sally (actually is a diminutive of Sarah)
Or since Elisabeth/Elizabeth is such a common middle name. Maybe something like Eliza for a middle name instead. Or Elise. Just some thoughts
I agree that Sarah is wonderful, classic, goes with anything (and therefore Atticus). Maybe if you concede Sarah, you can go just a notch more creative than Elizabeth for the middle. I really like Sarah Elspeth or Sarah Elise.
What about Clara Elisabeth? Clara and Atticus sound adorably vintage and Clara is classic without being boring.
I know Clara is a love it or hate it name, but it seems like a great substitute to me.
Sarah Elizabeth is the exact name that my husband wanted to use for one of our daughters. (Elizabeth with a "z" and not an "s" - but it's very close!) I think Atticus and Sarah sound fine together, but I'll list a few of the names that my husband liked as well as Sarah:
Didn't read the other responses, so sorry for repeats:
I'd try to talk him round to Sadie or something, if I were you.
No offence intended to anyone, but Sarah is one of the dullest girls' names out there, in my opinion. Elizabeth is not doing anything to liven it up, either. I'd feel a little sorry for a girl who got stuck with something as boring as Sarah when her brother was named Atticus.
I give another thumbs-up to Sarah! As others have said, even though Sarah is much more "ordinary" than Atticus I think they'd work together.