I just posted about umbrella strollers! lol My guys were 4.5 yo and we took them in umbrella strollers. It was a wonderful decision. They would not have fit in a regular stroller. They are tall, so we were pushing it with the umbrella strollers! Anyway, they were quite happy the whole time. Umbrella strollers give you the advantage of being easy to take everywhere. Like NotUrsula said, we would each put one between our legs on the bus. Very easy. They also are skinny and easier to maneuver in the crowds. Ours had a loud pattern that was easy to spot. Lastly, they were cheap enough that we had no trouble leaving them outside unattendend.
Ok, on the flip side, my guys stopped napping before age 2. I didn't have to worry about them sleeping. I've seen tons of kids fall asleep in umbrella strollers though. (Not to mention some other very strange, uncomfortable places. Had a friends kid fall asleep on my tile floor while we were talking.)
Is there any chance you could take him out somewhere and give it a try? For example, I took my boys to Fiesta Texas to see how they reacted to things before the trip. Maybe you could just try out a long mall trip, or a musuem or something?
Good luck!