ultimately, the program still depends on his current goals and the results of your fitness assessment.
The fact that he has played semi-pro fotball in the past has bearing only to the extent to which he approaches training in general. I have sometimes found that ex-athletes still want to be pushed beyond their limits, and it is often not easy to introduce a training approach more consistent with their current status and goals.
On the other hand, he probably has a good workout discipline and a great sense of body awareness even if his playing days may be long gone. That means that he needs to be challenged not necessarily with pure strength workouts but with those that involve the entire body using a lot of core strength.
I wish you fun and good luck.
I would concentrate on strength work, and as much “functional ” training as he can withstand.
TRX is great and Circuit Training would be beneficial.
Once you do your initial assessment you should then be able to design a proper workout.
I agree with Jason and also: unless you analyze and organize, it won’t work otherwise.