First, sorry to hear about your DS and hope he is better soon. It's too bad he won't be able to go on those rides, but better to miss a few rideds than possibly end up with a worse problem than a concussion.
You could skip the thrill rides or any with warnings (they are marked on park maps with a red triangle), but it sounds like you have others in your party who would not be too happy with that.
Some other suggestions I can think of off-hand:
I'd suggest first of all, that you use Fastpass. That way, your wait in line won't be long and he can wait outside for you without having to be sitting there alone for an hour.
Also, by knowing when different things are busy, you can avoid long waits. For example, the River Rapids at AK is very busy early in the morning, but even during Spring break, we have gotten on in the standby line without waiting at all - gone multiple times pretty much as fast as you can walk off and back on. Tourguide Mike (see the Theme Parks Board for a link to his site) has a lot of information on how to avoid being where it's busy.
I believe the cut off age for the Kids' cruises is 10 (although the page in the link that follows say 12), but the
Kids' Club Centers, located at many of the WDW resorts take kids up to 12. Maybe you could plan an activity like that for him and the rest of you try to do the thrill rides while he is on his own activity. People whose kids have used the Kid's Clubs have said they had a good time.
There are also babysitting services that come to your room or can take your child out to the parks, with guidelines to not go on any thrill rides (an 11 yr old might balk at that type of service though).