Though I don't understand the level progression thing...so, why does he need to borrow from another class to gain levels? He's got his skills modifier, his HD is 6, he gains new abilities. Sorry, I'm a newb, but a curious newb. How does it work?
If I'm understanding what you're asking, the PrC gives special abilities till 10th level. However, you have to have at least a few levels in a psionic class to take it and the original class and the prestige class sort of meld into each other.

In this PrC's instance you don't continue to gain psionic powers and psionic points every level, only on the levels indicated, but instead gain unique abilities native to the PrC.