That is amazing. Turning vegan could certainly help. Not consuming animal products from animals fed growth hormones is a really good start. But I would not think it's a cure. Keep us updated on his progress.
I was diagnosed in 2002 (PSA 6.5 GS=8) and immediately started on a (mostly raw) vegan diet along with juicing. My PSA immediately dropped. I had no treatment for 16 years. Consulted with several doctors. All said "You goona die!"
If you want a really fringe explanation, it could be the biopsy. Things like biopsies, radiation, or chemo can break up tumors and suddenly they become visible to the immune system which attacks the tumor. Its rare at best, and a quack theory at worst. But if you look up "abscopal response", you'll find cases where one tumor is radiated and tumors far from the radiation site begin to shrink.
I don't normally spread such theories, but being in chemo right now I'm hoping for something unlikely but highly helpful in addition to the normal effects of chemo.
With numbers like that I would be sceptical about the biopsy result. 1 out of 12 cores, gleason 7, stage 2a, would not typically produce a psa so high. PSA 23 I would expect Gleason 8 or higher and 10 out of 12 cores positive.
So it seems the psa was high for a different reason. You will need to keep monitoring once psa reaches bottom, then see if it shows a steady increase. Maybe try to get an MRI to compare to the biopsy results.
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Rising psa after Eligard and bicalutamide, in 9 months. What treatment now to stop the psa rise? 
New to group; very high PSA; headed for da Vinci surgery
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