I think you're accurate in saying he's a normal 5 year old boy with no behavioral problems. The circumstances you described of his experiences before kindergarten are pretty self-explanitory of his behavior. He is adjusting to a completely new environment that he is not used to being in, and it will take time to fully get the routines, rules, and behavioral expectations since he's had no prior experience. Some children take a few weeks, others a couple of months, but eventually, they adjust.
If his teacher cannot deal with his behavior, then she needs to tell you so you all can work something out that suits him better, such as switching him to another class. Not all kids are a one-size-fits-all in their behavior and experiences. It would have been ideal for him to be in a pre-school program, sure--but that can't always work out for every family and every child because each situation is unique. That should not be held against you or your son.
He will adjust eventually. His teacher needs to be the one who is more proactive in communicating a helpful solution with you if she's having so many problems with him.
I agree with the above post completely. I would recommend requesting a sit down meeting with the teacher. You can't really resolve anything through notes passed back and forth.
When you have a parent-teacher conference, you can communicate your concerns and opinions, and the teacher can do the same. You can then work TOGETHER to find some interventions that may help him to adjust comfortably.
With your son never being exposed to this kind of environment, and not used to being around other children, it's totally normal for him to be struggling to adjust like he is.
You and the teacher need to get on the same page to devise a plan to benefit your son.
Tbar, I agree with the others but I also have some real concerns about his formative years. You don't go in to great detail, but it sounds like he's never had a friend, and had to develop the skills of understanding feedback from other children, or conflict resolution, etc., that he should be learning at the age of two.
I don't think preschool is necessary, but ongoing relationships with children through playgroups, close relationships with same age cousins, neighbors, regular library story hour with the same core group of children each week, etc., are essential in developing his social skills.
It sounds like the only times he's ever been exposed to other children are brief, chance encounters with children he has never met and won't ever see again, in an outdoor park setting.
If this is an accurate picture of his childhood, he's going to need a lot more understanding and help learning the skills the other children learned when they were 2 and 3 years old. Children who have never been to preschool or daycare often thrive - usually even excel - in Kindergarten, but not if they haven't had any experience at all forming relationships with other children.
It sounds like you need to have a conference with the teacher and school counselor, if this is an accurate view of his childhood - that he played in solitary almost all the time - so he can get the help he needs.
And then move forward, fostering friendships yourself with mothers of children and strongly encourage him to develop after school relationships with children.