I have a lead male character who is very introverted and has strong opinions about...pretty much everything.
The problem is this: He keeps most of these opinions to himself because, like the title says, he is shy and well-mannered enough not fight with other people over things he finds objectionable.
It's actually been okay so far (considering it is written in first-person, and that the reader knows what he is thinking), but I'm still a bit fearful of him coming across as weak. Nobody likes a weak main character!
Any suggestions?
Is he the pov? It's easy enough to show how he feels without him actually saying things.
Tom forced his face into an agreeable expression. "Of course I want to go to the concert with you." A lie, of course, unless he miraculously lost his hearing between now and Friday.
Or you can use body language. Tom ducks his head and avoids eye contact, doesn't challenge someone who's being an ass. But his fists and jaw ache from being clenched.
And as for seeming weak. I don't think a character who is shy is necessarily weak. Weak characters are ones whose personality traits aren't developed or whose motives aren't clear. The term is sometimes also used for characters who lack agency or are just swept along by the plot instead of making choices that help drive it.
But two things to consider: one is that the transformation from passivity to greater agency and decisiveness is a really common (and popular) character arc. Complaints about characters lacking agency are usually made about characters who are this way for an entire story.
A second thing is, a desire to avoid conflict or be left alone due to being shy or introverted can actually be plot driving in itself. For instance, say Tom decides to go along with something because he's uncomfortable saying no to something, and he ends up smack dab in the middle of the story's catalyzing event, one that forces him to make choices and act.
And third, strength doesn't always show on the outside, and there are many ways for a person to be strong.