What other seats are you considering? If you can also get narrow seats for the other two, you'll have more leway.
What are the height/weight stats of your two kids? Do all three positions of the camry have shoulder belts and headrests (or at least the one your oldest will be using)?
You mentioned he is "sturdy" so that would be my only concern with finding a narrow booster. The narrowest booster I know of is the bubblebum, but that is an inflatable booster and most people don't use it for an everyday seat (backup or send to school, or extra for carpools).
Here is a spreadsheet with the booster measurements.
That said, when you're talking about puzzling seats together, the measurements don't always tell you everything! Especially with your son being "sturdy" I think you'll want to try him out in some to make sure they are comfortable.