I can tell you that a box-pleated kilt is more comfortable to sit on than a knife-pleated one, but that idea has already been discussed.
The sporran can be worn over the hip, as some dancers do, if it proves uncomfortable in its "usual" place.
Can the person in question pull up his own kilt hose and put on his own shoes? If not, that's a consideration that his caregiver will have to think about.
If getting a kilt doesn't work out, would a tartan lap rug be out of order?