Boy, you are speaking to me right now. This happened to my son who is older than yours but it happened to him with another couple of friends way back when. It hurts! I've been through a lot with my son. Being concerned is absolutely normal and your helping is important! For some kids, it all works out. For others, they need help. So, here are some thoughts. First, do a quick realistic analysis in your own mind of your child's social skills and social history. Has he had a bit of a hard time making friends or peer interaction all along? If so, what do you think the issue is in that regard? My son has sensory issues and being socially awkward is common with that. So, digging in to where you can help your son develop the best skills socially will always benefit. Next, the teacher changed the seating arrangement impacting your son. Have you talked to the teacher? Perhaps he could be seated with one of the other kids again in a non nonchalant so no one knows kind of way? Schools understand the social well being of students. It impacts a lot of things so normally, I've found teachers helpful in this regard. Also, the school may have some ways of helping a child in the need of friends get connected with other peers. Don't be embarrassed to talk to them about this! Your son is not alone!!
Does your son have any activities outside of school? If not, start getting him involved in something if you can. Scouts. Sports. Art class. whatever he likes! And then facilitate friendship situations. Make a space in your home for 'hanging out' for him and his friends. Allow him some gaming time if he likes that (my sons play x box and use the headphones to talk to other people). Invite someone over or take them to go do something. I know covid is a beast that has interrupted all socializing but with the world getting vaccinated, that SHOULD be getting better!
If your child seems sad and it lingers, or if his is a recurring problem of not feeling a part of things, please listen to what I'm about to tell you. My son has been in the same boat as yours. He's now 17. He was rejected AGAIN this year at school. He's always had some anxiety and it has now added to it depression. And it's severe depression. He is lonely. He feels worthless. His self esteem was shaped by rejection several times. Don't wait to get your son some psychological help if he is talking bad about himself. Because that could get worse. And can become a serious problem. Suicide is a real threat for teens these days with being the number way they die. So, not being an alarmist. But this type of isolation and rejection often goes hand in hand with kids developing a mental health crisis if they are prone to it. It sure has my son (heart breaking). There is no shame in therapy and if your child needs it or even 'might' need it, starting early is important. I wish I'd started earlier.
Stay in touch with me to let me know how it goes. I can tell you are a caring parent and I've been in your shoes.