Don’t make him get on the floor; there are a lot of ways you can hep him seated or standing.
I agree with the answers so far. If the pain is behind the knee, you might ensure that the knee is slightly bent.
You might also loosen up other areas along the posterior chain. I haven’t yet met a client of his age range with great foot mobility. I’m sure Karin would promote the MELT foot method, of which I’m also a fan. And then going up the chain into both calf groups to increase dorsiflexion.
Small increments, never to the point of pain.
If he can get into and out of a pool safely, I’m also a proponent of pool work and floating for this population.