Well with my kids... they are quite speak up types.
But, so is my family like that too. And my Husband. Me, more so than my Husband.
So that is our family.... personality.
BUT... also, from the time my kids were very young... I ALWAYS emphasized to them... to ALWAYS be themselves.... to SAY what they are feeling... and to say anything... right or wrong... and it is okay. I along with that, taught them HOW to say their feelings, the names for feelings, how to speak up and ask for help.... from me/a Teacher etc. And that it is okay.... to say things.
I/my Husband... also from a young age, taught our kids about what is right/wrong... and what a "bully" is... and what nice or mean behavior is.
If a kid took something from them at the park for example, they WILL say so to the kid... AND I have even witnessed my then 3 year old son even go up to the older kid's Mom and TELL her "your boy took my toy....that's not nice...."
BUT... I also, in front of my kids... really stand up for myself... or, when wrong doing is done to them. I then explain "why" Mommy is reporting something or telling the Teacher etc. I explain... my logic to them... so they learn to "discern" behavior and HOW to problem-solve it etc.
My kids, actually "see" me, being quite proactive... in correcting or clarifying or speaking up against something wrong... and I always explain to them why... that NO one... can harm them... and you cannot allow someone to harm or bully you....
If someone cuts in line... my daughter or son, will tell the kid. Or they tell me...and I will either say "well that kid is younger than you....its okay..." or if I know that kid was mean in doing so I WILL tap the child on the shoulder and say "my daughter was first.... we were waiting first...." so that my kids, by watching... will learn 'how' to handle situations as well.
To teach him to be expressive, it is either personality... or, you need to teach him that it is OKAY to say how he feels. If he does not know how he feels... then work on that with him... teach him the words for it and the actual ways he can say it. My 4 year old son for example, just today... was playing with my daughter. He then wanted to play by himself. He told her "I want to play by myself now....". (which is fine and good of him). But my daughter would NOT listen to him..and kept pestering him. So then his voice got louder and sterner and he said "GO AWAY. I said I want to play by myself.... !" So, I told my daughter... to stop it... that her brother SAID what he wanted and she didn't respect that. So she is the rude one. And that he expressed his limits... so she must go away.... and that everyone, wants to be by themselves sometimes. Its okay. He does not 'have to' play with her.
I always praise my kids... for expressing themselves... and articulately, per their age.
My own Dad... taught me how to speak up and stand up for ourselves.
If you want your son to express himself and to stand up for himself... it starts at home. Meaning.. you have to be willing.. to "allow" him to say his feelings.... good or bad or grumpy feelings. THEN praise him, and teach him 'how' to verbalize things in different ways.... so that given a situation, he can say something politely if need be... or STERNLY if need be. How a child learns this is from you and allowing him to try and practice it at home... and doing pretend play and practicing scenarios about it, with him.
If he feels he cannot say how he feels, or that he is not 'allowed' to at home... then he will not know that it is okay, to do.
If this is just his personality (I have a friend who's son is like yours)... then you need to instill a sense of self-confidence in him... and how to be HIMSELF.... so that he will know himself.... well enough so that he will not just go along with anything trouble makers do. AND teach him, anytime something is uncomfortable or something wrong is done to him... he has to tell you or the Teacher....
I know, not easy... but maybe as well... having him enroll in some kind of sport of activity martial arts is good as well. It is about the development of the overall child... and their sense of self-reliance.
all the best,