Train a 70yr old client twice a month, how do I track progress? only perform full body exercises for both sessions? Need help!

Jonathan is correct about how to approach your client. Since you only have 2 sessions a month with him, I would spend my time consulting him and educating him as to what he needs to be doing when you are not around. An assessment will help you come up with the right program for him and also keep track of his progress. On the other hand, if you see no progress with this client maybe adding a few more sessions during the month might be a better option.

Best,
Harris