Hi.
I just wanted to second what Cheshire Figment had to say.
He should remember to bring his hangtag. We did forget ours once and wrote a note on our WDW resort parking permit (the piece of paper that stays on the dashboard to show you can park in a WDW resort/park). That was a gamble, since we knew we could get ticketed or towed, but we could not do anything about it since DD had a non-folding wheelchair and there was no way we could park in the regualr lot and take the tram. We did take the bus a lot so we didn't need to worry about parking.
You could also check with the company you are renting from. When we had to rent a wheelchair accessible van for a few days after someone rea-ended my van, the van we rented came with handicapped plates since it was a wheelchair ramp van. If they say it has handicapped plates, I'd still bring the hangtag (sort of as insurance in case you get a vehicle that doesn't).