I used my Quickie 2 ultralightweight wheelchair on my last Disney World trip in January with my family, and friends. We used the Magical Express bus, and when it arrived. The driver told us that I had to board the bus like everyone else, and he would put my wheelchair into the cargo bay. This was no problem for me because I could do stairs as long as my back was OK with it, and I took them slowly.
That seems strange that the driver you had didn't allow the wheelchair to be stowed under the bus. Wouldn't that be an act of discrimination?
See my reply about the buses going to and from resorts and parks.
If you're still looking into getting a wheelchair for your son. The Quickie 2 is very light in weight, and only weighs about 27lbs. They make them in both kid-size, and adult-size models.
My mom likes it for the fact that she can lift it easily in, and out of the back of her station wagon without any trouble of it being extremely heavy as some manual wheelchairs are.
I think the OP is looking for lower cost wheelchairs since she posted she could purchase one for the price of 2 rentals.
Custom lightweight manual wheelchairs like the Quickie 2 generally start at about $1000 to $2000
For the OP, I would suggest looking at
www.adaptivemall.com
if you are interested in special needs strollers or less expensive (but still expensive) pediatric wheelchairs.
Look in strollers:
http://www.adaptivemall.com/allstrolpus.html?campaign=strtopnav
and in pediatric wheelchairs:
http://www.adaptivemall.com/concon.html
to get an idea of what is out there.
If you are looking for some other type of wheelchair, like a transport chair or less expensive one, this site is helpful:
http://www.1800wheelchair.com/home/wheelchairs
The transport ones (with 4 small wheels) are less expensive and also lightweight, but may be hard to push, especially with a heavier person riding.
I would personally advise against getting air filled tires. With air filled ones, you have to check them to make sure they are still full and may have to replace tubes. That happened several times when my DD had one with air filled tires.
Solid tires may be fine for someone who is not a full time wheelchair user of who can tolerate at rougher ride, but the are not the only option to air filled tires.
We have had pneumatic tires with airless inserts in DDs chairs for a long time. They are no-flat tires filled with gel; they give a similar ride to air, but without any of the issues.
The wheelchair front casters are always going to be non-air filled. DD had air filled ones on her first wheelchair and they were a real pain (plus expensive to replace).
Depending on what your child needs the wheelchair for and your insurance, you may be able to get much of the cost covered by insurance with a doctors prescription and documentation of medical necessity for the chair.