Ok, first off: I think I understand what he is trying to do:
He wants all your classes to progress simultaneously, instead of the 3.5 alternating method.
Assuming you still want to play with this DM, you need to do the following:
1. Try to convince him PrC's are not classes, and they have prerequisites.
Tell him the prerequisites should be met before you can take the PrC, and that when you take the PrC, you shouldn't be allowed to multiclass it.
2. Try to convince him his idea isn't bad, but the implementation is.
If he wants parallel progression, an X amount of XP should amount to the same total of class levels for single, double and triple progression.
To accomplish this, he should add the total levels of the classes, and look up the amount of XP it would take to get to that total level.
They shouldn't have to divide the xp over the classes, and it's easier to keep track of. (you can use the standard XP table,but levelling happens in jumps)
That way, dual or triple class characters start with an advantage, but stay at the same level longer, and all classes level at the same time.
Another way would be to allow dual or tripple characters to advance a class of their choice each time they level, with an LA of +1 for dual classers and an LA of +2 for triple class characters, and requiring all classes to reach the same level before continueing the highest level class.
(in fact, he will be using the normal progression rules, but requiring multiclassers to keep their classes balanced. You don't have to tell him that though

)