It might be somewhat normal, but that does not make it acceptable. Sounds like Kevin is definitely crossing a line and needs to be reminded how to be a better friend. We don't have to be best friends with everyone, but we need to treat everyone with kindness and respect. I would talk to the teacher about it, that you have some concerns because of what you are noticing and you want to bring it to her attention so perhaps she can address it - either directly with Kevin, or with his parents if necessary. If Kevin is not held accountable for what he is doing now, it may only get worse. If you can, you can even spend a day volunteering in the class and if you catch Kevin saying these things to your son or to anyone else, just tell him very firmly, "We don't say things like that to other people." or "We don't treat our friends that way."
I am all for kids learning to stand up for themselves and sort their own problems out, but at this age, they still need our help and need us to advocate for them. My daughter is 4 and is preschool and was having some issues with another girl in her class - not as bad, but similar. I told DD all the same things that you are telling your son. Her teachers were aware of it and would discipline the girl when needed, but I was still getting complaints from DD about how "Susie" was treating her. "Susie's" mom happens to be another teacher in the school and is fully aware of how "Susie" can be - she has basically admitted that she can be a brat sometimes and she has no problem with another adults correcting her if necessary. One time when it was my day to bring snack and stay with the class, "Susie" decided to be all friendly toward DD, wanted to sit next to her, etc. Since I was right there, it was very easy for me to whisper to her, "You need to be nice to DD - all the time." and that was it. Haven't heard a complaint since, and DD has told me repeatedly, "Thanks Mommy!". It just made her feel so much better inside, knowing her mom has her back. Sometimes these kids need to just be called out on their behavior. At one point a group of the kids were at a table playing with play-dough and one little boy asked "Susie" for the scissors when she was done with them. She told him no, she would give them to someone else, but not him. I was right there and very firmly told her, "No Susie, you need to give Jack the scissors when you are done. He asked you nicely and that's not nice to be that way." She gave him the scissors.
I would also think about signing your son up for karate, or tae kwan do. Might make him a little more "bullyproof" and more apt to stick up for himself, and for other kids that are being picked on.