If I played basketball for 30 minutes- I would be in hospital- let alone just having sore knees.
If you are going to the gym- Do a series of exercises that will strengthen the weak points- Knees- Cardio vascular and a bit of gentle weight training.
Cycling is very gentle on the joints-providing you build up time and effort gradually. Rowing machines at a low level will get the back- upper body and knees working- And something like a cross trainer will get everything moving without too much strain.
The best supplement to get him back to normal mobility is gentle exercise. Doesn't have to be that gentle but low levels on the machines- low weights in the weight room but with repetitions. Do it on a regular basis and just go up a level as required and it won't be long before he can get back to the basketball. That is if he doesn't buy a bike first and find out how relaxing and strenuous that can be.
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How long was I in the army? Five foot seven.
Spike Milligan