Both, my brother and sister are severe asthmatic adults (have been all of their lives). I was mildly asthmatic as a child and so was my dd. So - based on my experience, I would say that only you and your 12yr old son really knows what his tolerance level allows him to do. There are so many varying factors that I don't really know how anybody could ever really answer your question without knowing your son. Questions like, does he like thrill rides, does he scare easily, how easily is he "winded" - etc.
All of the rides that have any kind of turbulence, thrill factor, etc., are posted with warning signs at the entrance. You also have lots of time to look over the rides on the WDW website to perhaps ask more specific questions about rides that people can answer....???