Gold Mobility will have the most comfortable ride, as on the Victory 10 they use captains chairs, which most others charge extra for if they even offer them and they include fan, cell phone holder, cup holder and USB ports to charge your phone. Just be sure to get the appropriate size for his weight, either the Victory 10 or Maxima. They also include damage (most charge extra for this) and theft (most don't even offer this) coverage at no additional cost. They also provide ponchos. And in general are less expensive than their competitors, unless you have a shorter trip than about 4 days, as they have a flat rate for the first 7 days. Their equipment is also kept in good repair and replaced frequently. They also will come to you in the park in the rare case there is a problem, if you rent from anyone else, be sure to ask them what they do, as some make you get it back to your resort or to the front of the park. They also will meet on your schedule, usually they will be waiting for you as you arrive to your resort or for many non Disney resorts they will drop off at Bell Services, many others have preset times depending on your resort. Their controls are also much easier to use than some of the competitors (obviously if the competitor is using the same models, then that isn't an issue, but some use different ones). They also have the best customer service of just about any company we have ever dealt with, mobility or otherwise.
3 wheel is the easiest to maneuver and have never felt unstable on them. There are actually a lot of rides that are ok to do, even with back concerns. One other thing to note is that if the seats may be a concern (for example, Small World Mya be an issue due to the height of the seat back, depending on where his back injury is) and the attraction allows people to remain in wheelchairs (again small world does this), they will usually have the option to transfer to a park provided chair and use that, just politely ask.
