I find stiff carbon boards to be a problem for me. Wood is much easier on my knees and hips. Carbon boards with extra foot padding feel better. TABOU 3S for example.

Are his hips/knees stuffed as result of injury or arthritis. Only asking as my hip is stuffed and I'm getting a new one -
old man style but latest imported technology!

I'm wave sailing & SUP surfing with some pain and restricted movement but the pain is usually worse after.

I plan to recover to full wavesailing before the NE's disappear for the summer.

...break a leg

I have a bad hip, it does not effect windsurfing much at all. Cannot speak for bad knees, maybe a problem when learning and doing a lot of crawling onto the board.