Beau & Joe had a fight...

[Faye]

This has come from nothing! they were so happy together - always sleeping together etc.
Then this morning, there was serious persual - Beau (daddy) chasing and biting Joe (son), Joe isn't retaliating, just squeaking loads and trying to get away... I've separated them by sectioning Beau off in one corner of the cage - I don't know what to do now though... is this just a dominance tussle that'll sort it's self out? or will I have to separate them permenantly?
there are no open wounds, but Joe's belly and rear is wet and looks sore.
They're ok with each other when I'm holding them both, but as soon as I put them back in the cage, it all sets off again
Beau is 16-17 weeks old & Joe is 8-9 weeks old
Please help!