It’s not overly dramatic, these early bad experiences can establish patterns for how he relates to his peers, and how he feels about himself. It’s very sad.
Try not to let your worry and concern show through, as if he is sensitive and deeply thinking he is going to pick up on your anxiety. Be very careful how you question him about school. Instead of asking about work, ask about his friends, ask for stories from his day where he was kind or brave enough to ask to play with someone new, or where he took the lead in a game, ask for funny stories of silly or naughty things other children have done. If he wants to talk about what has not gone well, let him, but don’t ask - wait for him to volunteer. And then help him find positives in his day - there is bound to be something if he looks hard enough. And encourage him not to listen to people who say horrible things to him - if they are horrible, it says more about them than about him, doesn’t it?
I would have a quiet word with the teacher about the teasing too, and mention your son is really taking it to heart and see if teacher can ensure the class don’t treat your son so badly.
I would encourage your son to escape into activities he loves - perhaps he enjoys music and could learn an instrument, or art, or football, or swimming, or baking or whatever. Building his confidence in an area where is naturally comfortable even if he is not brilliant, will make make him more likely to succeed, and build self esteem.
Also maybe find some stories of people who have not been successful at school but then gone on to do great things. Winston Churchill had to repeat a year at primary school, Thomas Edison was kicked out of school age 12 for being too poor at maths to continue and told he was “too stupid to learn anything”. Dylan designed over 5000 vacuum cleaners before he found the design that would earn him a billion dollars. He doesn’t have to be perfect right now, and all this resilience he is learning now will make him a stronger, more determined person in the long run.
I really hope things improve for your little boy.