I agree with Cheshire Figment's comments.
In most of the lines, you are not standing still that long. You are either slowly moving forward or standing still for a while then moving forward a lot at once for some of the attractions that load in 'shows' or 'batches'.
If you do elect to bring something to sit on, it needs to be easy/fast to put up and down. There are some canes that have a fold down seat
- like the ones on this page. In most cases, he would be able to bring a seat cane like that right on the ride with him. For some of the attractions that might not have room for the cane, he could leave it with the CM at the boarding/exit area.
Another thing to think about is whether or not his knees would cause problems with boarding - especially for those attractions with moving walkways. If you look in the disABILITIES FAQs thread, there is a link to a past thread about 'Boarding for someone who needs a bit more time.'
Also, he may not
want to use a wheelchair or
ECV, but if he can only walk in small spirts, he is likely to have problems at WDW. And, the queues are just a small part of the walking at WDW. The average guest walks between 3 and 9 miles
per day at WDW. Just to walk around the lake at World Showcase is 1.3 miles!
One way that has worked well for other people is to figure out a familiar place that is about 2-3 miles from his home.
Ask if he could walk there.
If he thinks he could, ask him about walking back.
Every day for the number of days you plan to visit the parks.
Everyone I've suggested this to has convinced their loved one that they need to rent an ECV or wheelchair. Using a wheelchair allows the person to use their energy for having fun rather than getting worn out just walking around. Rather than using it all the time, some people choose to use a wheelchair for getting from place to place and then walk into shops and rides.