Hi,
My youngest son will be 13 soon. He was in Catholic School form Pre K till 3rd grade. I took him out and put him in public school starting in 4th grade till now he is in 7th. He hated it

it took him a good 8 months to get used to public school. My 2 oldest always went to Catholic school and graduated from there. The public school system in New York is way ahead of the Catholic schools, my son was lost, but he did catch up in about his 6 month.
I don't know for anywhere else but her in New York the school system is just so more advanced than the Catholic school. In 6th grade my son also got very quiet and just distant, I knew something was happening.
Finally one night he broke down and told my husband he was being bullied

this was going on for 6 months and he didn't tell us.
The kid that was bullying him was 5 foot 8 and 180 pds? My son then was barely 70 pds and just under 5 feet.
He showed us his legs and they were all scarred up

this kid was taking his size 10 foot and kicking my son , leaving in prints in his leg. At gym he would pull his shirt over his head and punch him and kick him in his back.
I wanted to kill this kid. I was so afraid he would hit him so hard that he might actually kill him or do serious damage.
I went up to the school and I was crying ( I didn't know at the time, but they are big on Bullying and take it very serious) I told the principal if something was not done immediately I was going to the mother and telling her if my son was touched again I would be coming back to her house for her.
The kid was suspended and he did never touch my son again.
Not saying your son is being bullied, but just make sure it actually isn't going on now at this moment. Kids stay quiet till they can't take it no more, just make sure nobody is picking on him or saying mean things to him.
I am sure as your son gets older things will get better, hope it is nothing and I hope your son will be just fine..
I don't think putting him in another school will help, I wanted to immediately pull my son out of school and put him back in catholic school. My husband would not let me, because he knew the education was not the same , so after a bit of thinking I decided to let him stay. I just watched him a lot..
Good Luck whatever you decide to do..