Introduction JB
I ponder
Jb on Seth and your concern of his speech. I get a strong sense his lack of talk is like you stated a compensation for Filipe's over talkative nature. My oldest and second is very similar in that regard. We still struggle trying to break that shyness.
My cue that all was really OK was watching my youngest interact when the oldest was not nearby by and nor was his mother. He would then use short as possible sentences to get his message across. I am hoping you are seeing this with Seth.
I often joke god has a good sense of humor as he never puts two in the same house with the same personality just to ensure we learn how to deal with dislike people.
Some how getting him alone to do more reading and descriptive chatting about his day might help break those barriers down for you it help for us.