From what you describe, he may now be castrate resistant - After a year of apalutamide and the Eligard shots with Stage 4 Prostate Cancer...the hormone therapy isn't working anymore..."

You ask two questions, about chemotherapy and what is going to happen next...

The forum is responding based on the clinical information you provide, more would strengthen the responses - Gleason Score, Grade Group, has he had surgery, if so, the pathology report, treatment timeline, age, overall health, any co-morbidities

Generally, chemotherapy works to reduce the cancer burden. I did six cycles of Taxotere starting in January 2017, I was 61 and in pretty good shape both physically and health wise.

I assume they determined his ECOG status:

The Eastern Cooperative Oncology Group (ECOG) Performance Status (PS) scale is a tool used to assess a cancer patient's ability to function and care for themselves:
What it measures
The ECOG PS scale measures a patient's ability to perform daily activities, walk, work, and care for themselves. It also describes the patient's symptom status.
How it's used
The ECOG PS scale is used to help guide clinical decisions and prognostication. It's also used to select patients for clinical trials and standard cancer treatments.
How it's scored
The ECOG PS scale uses a 5-point scale:
PS 0: Normal activity
PS 1: Some symptoms, but still near fully ambulatory
PS 2: Less than 50% of the day in bed
PS 3: More than 50% of the day in bed, but not bedbound
PS 4: Completely bedridden

I did lose my hair, it grew back, a few of the nails turned black and blue, they came back, I had chemo mouth but ate pretty much normal diet. I did continue to work, went to the gym, did yard work, rode my bike...My infusions were on Thursday, with the anti-nausea medications they put in first to prevent vomiting from the Taxotere infusion, I felt "pretty good" the first 24-28 hours, then would crash for the next 72-96 hours, then feel better for the next two weeks and start over again. The side effects are cumulative, the fatigue hit hard on the 4th infusion.

As to what's ahead, the possibly many treatment choices to manage his prostate cancer (see the article I attached). If you have not already, familiarize yourselves with those. Reading through the NCCN Guidelines for Advanced PCa and the Prostate Cancer Foundation which has excellent resources can inform you of those treatment options so you can discuss with his medical team.

It's not over!
https://www.nccn.org/guidelines/guidelines-detail?category=1&id=1459
https://www.pcf.org/
Kevin