K.B.
I would agree that he may turn things around once he is in school, and they can be very different kids at school and with teachers than with us at home. My daughter can have melt-downs with me at home because it's her "safe place" - stuff she would never pull at school because she knows she needs to hold it together. You can talk with the teachers and school counselors if it continues. And sometimes "natural consequences" (i.e. realizing other kids won't want to play with him because of this behavior) work too, and "peer pressure" can be a positive thing.
Realize that your son may always have this kind of personality and it may just be his nature to be a perfectionist - my brother was the same way, it served him well in college and in his career (he is an engineer). As he matured, he got better about winning vs. losing, and not always getting his way. However, if your son chooses to throw a temper tantrum, with the kicking, the tears, etc., I would not tolerate it - I would send him to his room and have him stay in there with the door shut until he decides to calm down.
I am starting to wonder too if the summer before kindergarten tends to be tough on all kids, because it is such a huge transition and a milestone for them, and in a way, they sense that and sometimes have a hard time coping with it. I know my daughter is turning 5 next week and starts kindergarten in 3 weeks and seems to sometimes just flip out and lose her mind over the smallest things. She says she's excited about kindergarten, and she doesn't appear to be anxious in any way, but I still wonder if it is still weighing on her mind a little.
When my daughter is upset about not getting her way, I sympathize with her to a point, so she does feel heard and she feels a little more understood, but I still discipline her for screaming and other over-the-top, unacceptable behavior.
Remember too to remain calm yourself and try not to let him pick up on your anxiety - because then it can become his anxiety too.