He sounds a lovely boy.
Being young can be very hard.
My son {now over 6 foot and an adult} used to be small sized for his class, and young looking {July birthday so youngest in his year}
He too was bullied.. but lost it and actually punched the bully.
It got him and the bully suspended for a few days...but worth it, as he was never bullied again .
It is scary being bullied, but if he can find the strength to stand up for himself {punching probably not a great idea in retrospect!} he will be respected more by his classmates.
It's horrible but kids can be like ''animals''..and they prey on who they perceive as 'weak'.
Your son obviously has strengths, and he won't always be unhappy.
Being a tween/young teen is horrible.
At a school reunion, years later, he towered over the bully , who actually apologised and said ''I don't know why we picked on you''.. Best of luck to your son.