AFAIK Not unless the rebels are deemed to be a terrorist organisation - which is unlikely as the way things are going they're *hopefully* going to form part of the next govt over there. FWIW the KLA openly recruited in the UK when it started kicking off in Kosovo.
It would depend on whether he can be classified as a mercenary, in which case he can be prosecuted on his return.
I doubt it however as he has dual nationality.