I don't have direct experience, but a couple of friends have a Convaid. They have really liked their strollers. Their kids can and do walk so they don't need a wheelchair but need help on longer treks.
Note that the EZ Rider has an option for Transport to make it safe for tie-down on buses; that is optional at the time the stroller is ordered and significantly bumps up the price; I don't know if it can be retrofitted. If there is interest or need in using the EZ Rider on buses, be sure it is purchased with the Transport option.
As to WDW and special needs strollers...reports vary. Since the vast majority of special needs strollers are not equipped for transportation use, it's my understanding the general rule is that all strollers must be folded on WDW buses. How that is enforced in practice may depend on the bus driver. I guess my point is that even if he gets the EZ Rider Transport, you may still be asked to fold it; if transferring is a problem for your grandson and he needs to remain in the stroller, you may have to explain it is transport-ready and point out the tie-down points.