Oh I am sorry, We want our kids to always be happy. This is an adjustment for him. He is feeling a little lost.
Our daughter is very shy. At this age she could not even ask to play with others.. So the fact that your son asks is great! For next time remind him when some says I am already playing with someone , you son can see, lets all play together. Also ask him to look around for others that look like they are having fun and ask if he can join them.. Not to be discouraged.
Kinder is more difficult than daycare because they are not Just playing. They are having to remember new rules, new schedules, and a whole new group of people. It takes a lot of energy to stay focused all day. He could be super tired. Make sure he is getting plenty of rest,
Maybe in his lunch box if he can read send him a note.. or draw him a little picture. Ask him, who did you sit next to today at lunch? What was in their lunch?
When you do speak of school, play up the good parts. Ask questions like, "what story did your teacher read? What did you do after lunch? What play area of the classroom did you get to play in?"
Also see if you can get a schedule of what their day is, so you have a better idea of what all he is doing and learning. It can guide the conversations.
He is going to be fine. It is just a big change for him. He is going to find his groove soon. Just stay positive.