I think the best things you can do is keep lines of communication open, talk with him often, let him know that he needs to talk (not shy away from) kids who are "not nice" to him.
Our oldest daughter was a "young 5" when she entered K too. She was very "handsy" and that immaturity was the ONLY thing making it obvious that she wasn't the same age otherwise with the other kids.
We did deal with some not nice kids, but she did fine. She learned to stand up for herself, tell other kids to leave her alone and how to deal with them.
The bottom line is I don't think you can "bully proof" your child. I think you can teach them skills on how to deal with bullies/mean kids, you can talk with them often about "how's school going", "who are your friends", and we played "roses and thorns" at dinner time - you say the highs and lows of your day.
The next best thing you can do as a parent is try, try, try to have him make some friends and have them over for playdates, join the same soccer team, and make friends with the mom of his friend. FRIENDS stick up for each other, FRIENDS stick together. Try to not make Kindergarten a new, scary place. Talk about how much fun he's going to have, about seeing his friends everyday, etc.
We've made some friends that haven't panned out, due to different interests, finding new friends, etc. We don't focus on "losing" those friends. We just say, "OK, so she has someone else she likes to play with. That's ok." And then I schedule playdates with the kids that my child really does like, play with on the playground, etc. I work hard to make our house the "funnest one" to come to, so that my daughter gets lots of playdate invitations in return. Now we have several "go to" friends that also play soccer, joined Brownies, etc.
If you are worried you will project that onto him. Try to be upbeat and focus on friends and fun and you'll both be just fine.
IF you notice a problem or suspect bullying, get involved with the bus driver, teacher or school principal right away. Don't wait for it to get better. Don't make excuses for your child being "young" or "sensitive". There's nothing you can do about those things. Focus on how the children treat each other and ask for their ideas and suggestions to help everyone get along.
Best wishes
Been there