Can he effect them as their nation (if they are English)?
Whether or how much Eugene counts as English is really up to you, but I figured it bore commenting on! He's a Canadian citizen, but he's been in England for years, married an English citizen, and settled possibly permanently on one of the Channel Islands before arriving at the fleet.

Can he use magic on them?
Absolutely! Dude comes from a non-magic canon and has zero magical ability himself, but that only makes breaking the laws of physics in his presence that much more enjoyable.