Sorry to hear about your son's misfortune.
Dolphin Swim: I'd suggest you contact the Dophin Swim people and also your doctor about that one. It obviously won't be fresh water, so the doctor may not want him doing it even with the vacuum cover. I don't know if the dolphins would have any objection to the cast, but I'm sure the people who care for them would know.

They have probably run into the question before.
Park with a leg cast: As Cheshire Figment said, you should contact your doctor to see if there are any special things your DS should avoid. But, if you go to the
Official Disney Website, Disabilities Page, there is a FAQ about rides you can go on with a leg cast. That site won't let me link that page directly, so here's how to get there. Follow the link above, then choose WDW. On the next page, choose FAQs. One of them is a listing of which rides/attractions have enough room for a full leg cast. Some might not be appropriate for your DS because of his particular injury (like they list Star Tours and Body Wars), but at least you know there won't be trouble fitting into the rides. You can always ask the CM at the rides. Since your DS is only 7, he may fit in some rides where an adult with a leg cast won't fit. If your DS has only a short leg cast (to the knee), there are more things he will fit into. There are FACs listing which rides you can stay in a wheelchair for, which you have to transfer out, etc. If you go back a page, choose an individual park and then
List of Attractions. That will give you more (not a whole lot more, though) information on descriptions or rides and whether boarding is by a moving walkway. They can stop the moving walkways (or at least slow them to a crawl) on all rides except Peter Pan.
Wheelchairs You shouldn't have a problem renting a wheelchair from the parks, but here are some other things to think about:
Distance: It's usually quite a long ways from parking or bus stop to the rental area.
Size: The wheelchairs they rent in the parks are adult sized. The seat length is too long for most kids, so they end up slouching down in them to get their knees to come at the edge of the seat. The armrests come up near shoulder height for most kids, which looks pretty uncomfortable to me. He may need to actually use the armrests to help push himself out of the wheelchair when he needs to stand and they will be too high for that.
Resorts: Getting around the resorts also includes a lot of walking and if you rent a wheelchair at the parks, you won't have one for other trips and at the resort.
There are medical equipment rental places that will rent a wheelchair for your entire trip. They will deliver to the resort and should be able to deal with your needing to have delivery at one resort and pick up at another. They would also be able to rent you a wheelchair that is more appropriately sized to a 7 yr old. Here's a list:
Care Medical:
http://www.caremedicalequipment.com/
Walker Mobility:
1-888-726-6837
www.walkermobility.com
RANDY'S Mobility is in Kissimmee 407-892-4777
http://randysmobility.com/
Colonial Medical
http://www.colonialmed.com/about_cms.html
http://www.all-about-kids.com/main.htm
You might want to look into renting in your home area and bringing the wheelchair with you. That way you can would have one in the airport. You can gate check a wheelchair so it would be available until you board. A wheelchair doesn't count as a piece of luggage - no extra charge for transport.
Also bring whatever pain relief your doctor recommends. DS might not have a problem with pain, but it's better to be safe than sorry. First Aid in any of the parks is a great place to go if he needs to lie down for a little while to rest (easier than going back to your room). There are RNs at First Aid who can help with questions.
Good luck and ask anything else you can think of.