My daughter was in kindergarten at 4.
I took her to school every morning, but she rode the school bus home. It dropped her off right at the corner and I was fortunately able to be there to meet her at first and then watch from the driveway to make sure she came straight down the street. She was never out of my sight, but she did just fine on the bus. She did great in fact and loved it. My daughter was really tiny, but she sat in her seat, the bus driver didn't take any goofing off from the kids and she knew if she didn't follow the rules, no more bus.
She hated having to ride the bus when she was in high school, but she was a seasoned veteran by then and I didn't really worry about safety.
As moms, we ALWAYS worry, but I would give it a try. Your little one might do just great. If he doesn't, then you know you have to make alternate arrangements, but for now, you're worrying a bit in advance, in my opinion.
Over the summer, I would get on a transit bus with your kids. Find a destination. Even if it's just to go and get ice cream somewhere and walk around for a while until you catch a bus back home. It can be a fun day trip and you can talk to your little one about how to sit down, no getting up, no opening windows, no goofing off.
I used to take my kids on the transit bus in my area all the way to the northern end of the line where my mom would meet us and we'd spend the day with her and ride the bus back home. I could have driven, but it was an adventure and we saw lots of people and places that you don't get to see right on the highway. My son is 14 and he's been riding the bus by himself for 2 years. I always put him on the bus and tell the driver where he is to get off and there is always someone waiting at the other end to pick him up, but all our practice paid off as far as him being responsible.
A transit bus is different than a school bus, but you can still practice with your little guy before it's his time to try the school bus.
I think once you feel sure that he knows the safety rules ahead of time, it might alleviate your fears.
Best wishes.