For the OP, your insurance should pay for a special needs stroller (what those big ones are called) for you. You might not be able to get all the red tape taken care of to get one in 4 weeks though. Here is a web site with a great selection:
www.adaptivemall.com
I can't get on the site where I am, but you want to do a search for "Special Needs Strollers". Maclaren and Baby Jogger make nice ones, especially if he doesn't need a lot of positioning.
You might want to try a jogging stroller to see how well he can get out of it; the recline and position of the wheels right in front of him might make getting in and out very difficult for him.
Check with an OT or PT about whether there are any lending closets for equipment in your area (if he doesn't have Ot/PT outside of school, contact the school. Even if he doesn't work with one at school, they should have one they can use for consultation).
I think there is a link in the disABILITIES FAQs thread on the disABILITIES Board re: a company that does rent special needs strollers.
For other people, the strollers at WDW are large and can carry up to a small 12 yr old (at least that's what some CMs have posted). For the OP, those strollers would probably not work well, they are sort of low to the ground and he may not be able to get a good position fro his arm crutches to get up from them.
A last piece of advice, you might want to look into a lightweight pediatric wheelchair. Even after he can walk well, there may be times when there is too much walking for him. A 4 year old in a stroller or special needs stroller has to be pushed and is dependent on you to move him. A 4 year old who can walk with arm crutches is strong enough to propel a wheelchair - not all day, but enough to give him some independence and a feeling of freedom.