He is totally enjoying looking out the doors, then he throws up out of the blue all over place on a thankfully failry empty bus. Yep motion sicknesss I guess.
My two younger children have extreme motion sickness (vomit every time they get into a car since they were 8-10mos old). For our first Disney trip, I brought "barf bags" for the busses just to be safe, but in 10 days of using the busses multiple times a day they somehow never got sick. (Any time we got in the car they did, so for us the busses were much more pleasant

) On our subsequent trips I didn't feel it was necessary to bring bags for the bus.
This last trip my parents went with us. About a week into the trip, we got onto a really full bus, so my mom sat up front with my youngest while the rest of us stood in the back. Just as we pulled into MK, he vomited all over the floor. My mom said you couldn't believe how much space people manage to find on a packed bus once someone starts to puke.

She kept trying to assure the guests that he has motion sickness and it wasn't something contaigious that would ruin their vacations. She later told us "He was just sitting on my lap reading this book and then he just vomited"... I have no idea why she thought giving a motion-sick child something to read on a crowded bus was a good idea
On our last day we were on a very packed bus to MK and DS and I were in the far back corner. We were sitting, couldn't see anything, and it was so hot and stuffy you couldn't breathe. I was getting super sick and then DS said he was going to vomit. Everyone just leaned away and DH whipped his hat off and held it out for DS to puke into.

We did manage to get off the bus with a clean hat, but my dad was laughing forever at the movement of the crowd and DH's reflexes.