I had a similar situation with a junior dev in our team some time ago. He would only ever contribute a sentence or 2 into any meeting when asked upon, otherwise

I had a similar situation with a junior dev in our team some time ago. He would only ever contribute a sentence or 2 into any meeting when asked upon, otherwise would be silent. We discussed it and tried a few things, but ultimately what worked for him was to write down some bullet points about what he wanted to say before a meeting. This gave him time to do the thinking up front and relaxed him greatly. Turns out he was really quite clever and chatty. It can just be very intimidating to think and talk on the spot amongst a group of experienced peers.