I have taught over 10 yrs as a sub in the K-5 level.
This is a tough adjustment period and a lot of children go through it. It is a huge change for them. They are learning the ropes of being in school, on a schedule, new faces with children and teachers, and a lot of change.
Continue to send him and don't change anything. He will get adjusted and be just fine. Reassure him that you will be there after school, you love him and you are excited to hear about what he does today.
There are some parents who just cannot walk their children to the room due to the separation anxiety OR volunteer and help at lunch or in the K area due to the anxiety.
Don't specifically ask "how was your day" because you get some typical answers, such as "fine". Ask about the friends in the classroom, the teacher, the books they read, the library trip, music class, art and PE classes.
You should be aware of the curriculum so use that as a guide for questions to ask.
In 1st grade, where I am the most, On Mondays, we staple a sheet to the daily folder which has what we will be doing all week, including the Wednesday homework, spelling pattern for the week for prep on Friday spelling tests, and a spot for a parent signature (they are "rewarded" for being responsible to bring in the signed folder daily), and a spot for any quick teacher notes to parents. At the end of the day, we go around and stamp the specific day with a fun stamp or sticker so they show mom and dad the folder and get the signature.
Try to make it fun and adventure. He will adjust, just give it some time. You already know he is participating in class and doing well with testing.
Best wishes to you.