Good lord! I don't think chemo would leave him bedridden ... some men on here have continued working right through it (although they weren't 84). And dying of prostate cancer is probably more painful than the side effects of chemo although both should be made bearable by good use of pain relief.
If he goes for one or two chemo sessions and finds it too difficult, he can presumably stop and who knows, that little bit of chemo might just slow it down a bit.
Have they done scans to see where the spread is? Can you tell us? Small cell sometimes goes to lungs, brain or liver rather than bone or lymph nodes, and that makes a big difference to how he might get on.