needless to say, you need a physician’s approval. I also assume that you do some form of assessment which you should do as for any other client.
Chemo may make him more tired as it progresses, in which case you will need to adjust the workouts. Since he has lost quite a bit of weight and has not exercised much lately (except for golf) I would start with a lot of stabilization exercises for core, hips and shoulders to lay a good foundation. I would recommend the initial goal to be one of strength endurance before increasing the weights to elicit hypertrophy.
As an experienced trainer, you should just follow your instincts and adjust the workouts as necessary.