Something along the lines of...

[Lauralastic]

DH is Alexis, SIL Tanith, DS Jago. Expecting 2nd DS in 4days; any suggestions please? Surname starts with N so any first name ending in N seems to sound wrong.

not sure about tanith,

alexis is ok

hmm,
diego

Duncan or Adrian. I know that neither of them adhere to your N ending rule, but they are the best names. Fact.

Rafe
Silas
Seth
Inigo
Amos
Eli
Solomon
Emrys
Sacha

Something a little unusual but not way out there...

How about
Huxley
Mallory
Caspar
?

Cormac

Luca/Lucas

Emile

Milo/Miles

Xander

Otis

Rufus
Rafael
Rhuari
Curtis
Basil
Bruce
Tomas
Cosmo
Oakley
Brock
Conrad
Apollo
Artemis
Archer
Dill
Fabian
Gabriel
Iago
Ivor
Hugo
Horatio
Kai
Casper
Leander
Milo
Quinn
Tristan
Yves
Zach