It is like two-weapon fighting, except there are more than two weapons.
You start with -6 to the primary weapon and -10 to all secondary weapons. Multidex reduces the secondary weapon attacks by 4 to -6. Multiweapon-fighting reduces both penalties by two, unless all secondary weapons are light weapons, in which case the penalties are reduced by 4. So the total penalties are -4, or -2 if all secondary weapons are light.
Since the secondary two-handed weapon is not light, the penalty is -4 to each. As for the strength bonus to damage, I would consider how it works if he used 4 weapons. He gets his full bonus with the primary weapon and half for the 3 secondary weapons. That is a total of 2.5. So give him 1.5 times his strength bonus for the primary weapon, and full strength for his secondary weapon, for a total of 2.5. This would simulate well how it works for two-weapon fighting and two-handed weapons for those with just two arms.
