My first question would be how tall is he and how big?
The type of wheelchairs that you can rent at the parks are similar to the ones on
this link. They are heavy duty commercial wheelchairs and are adult sized, very heavy (40-50 pounds).
The seats and backs are vinyl sling type, so they are not firm to sit on.
If your child is close to adult size, the size might work out OK, although some people do not like the 'un-firmness' of the seat (I can't think of a good 'real' word) and some don't like the width, For example, my DD is 5 feet tall and 90 pounds with a slim build - her wheelchair is 14 inches wide; a typical adult wheelchair is at least 16 inches wide, but more often (at least these days) they are 18 inches.
If he is shorter, the armrests will be too high (anywhere from mid-chest to neck) and the seat will be too deep, which means that if he sits all the way back so his back is supported, the edge of the seat will come below the knees. If he sits all the way forward so his knees are at the edge of the seat, he would not have any back support. Those things bother some kids with autism a lot and don't bother some kids much at all.
Another possible option would be to rent a special needs stroller (there are links to places you can rent them in the disABILITIES FAQs thread). Those are basically like giant umbrella strollers and many children with autism have those strollers.
You could also consider renting a park stroller. They are made of a firm plastic and many people have posted in the past that their children with autism like them because of the firm feel and also the canopy and sides that cut down on stimulation.
Allearsnet has a page about strollers, with pictures. Up to a small 12 yr old will fit into the strollers.
When you go onto rides, you can bring the wheelchair with you in line (you would get a Guest Assistance Card to bring a stroller into lines - see the disABILITIES FAQs thread for info about GACs). You could also park the wheelchair/stroller outside of the attraction in the stroller parking area.