Does he have good writing skills? Wonder is an internet research company that allows you to work as few or as many hours as you want, and I find it pretty fun. You have to apply and do a sample research request, and there's a bit of a learning curve, but it might work for him. He'd need to stay below the disability earnings limit, obviously.
I have been in a similar situation myself, and I did a lot of knitting while out on disability. I found it relaxing and self-paced.