Hello,
Great to hear that his PSA has fallen so much so quickly, that should hopefully provide some reassurance, and long may that continue. Just thought I'd chip in with a few reflections.
The emotional side effects are most likely simply the result of the treatment, which is known as 'androgen deprivation therapy' (ADT) - the intention is to suppress testosterone to very low levels, which tends to create those emotional side effects. So unfortunately changing the type of ADT would be unlikely to change these particular effects, they're probably all going to have the same effect, though feel free to double check with your oncologist (after all, I'm not a doctor!).
A number of clinical trials show that upfront chemo can be really beneficial for some men with advanced/metastatic prostate cancer. My father was diagnosed around a similar time last year (late July) and started chemo in early September. His scans from July last year showed a huge number of tumours all over his torso, but by February/March his PSA level was untraceable (<0.01) and his scans indicated a complete response with no tumours visible in his bones at all, and a very small number of cells remaining in his prostate. So his treatment was highly effective in his case.
His PSA has started slowly creeping up since then (about 0.4 a month ago, which is still fine if it stays there, but worrying if it keeps rising), but certainly I think you can trust your oncologist's recommendation of chemo at this stage. Hope all goes well.