If your opponent has monsters in face up defense position and he summons one in attack position and attacks, if I activate Staunch Defender does he has to attack with the ones that are in face up defense position?

No he doesn't, the only monster which will attack are the ones tha CAN attack, meaning monsters in Atk position.

Thanks a lot for the quick reply.

Staunch Defender should not be confused with Amazoness Archers (which does specifically change positions of monsters and, since they're now able to attack assuming they have no summoning sickness, they have to attack).