My Son is serving a 16 month sentence in state jail, is there any way I can find out if his sentencing judge participates in the diligent program? And if he gives the full 20 percent?

The judge has nothing to do with the allocation of good time. The good time credits are set by the State's Justice Department. If the program you speak of offers a sentence reduction for successful completion, tit doesnt matter what the judge wants, they hold zero power over the Department of Corrections as they follow their internal procedures. I know this sounds harsh, but a 16-month sentence is a joke compared to the time we've all done - he'll be fine no matter the outcome of the possible early release.