You’ve received great insight from the others on exercise and nutrition starting points and things to consider.
I would add this, be sure to lay out clear, easily attainable goals.
He has probably tried losing the weight before and not been able to. He’s probably not even sure working with you will change that. After all, all his experiences have taught him that he CAN’T lose weight.
Find out why he wants to lose the weight. What has he tried before? Why has t it worked? Why does he think this time will be different? What does he think he may have to give up or change to make that happen? Is he willing to do that? How? What will make this time different? What will he do differently? And what can he do today to get closer to that goal?
It’s as much about understanding his motivation and his obstacles, as it is what he wants to achieve and knowing how to get there.
Enjoy the learning process. I often find that I learn as much from my difficult clients as they do!