If you have enough of a bond, I'd have a chat with him. I'd explain that as a smart person and valued member of the team his verbal output would really help. I'd say his perspective would be great to hear more as you know he has the skills to have good judgement. Obviously this would need tailored to the person at hand but, I have had good results with this method.

Ultimately they may never be the most chatty member of the team but some output is far better than none and encourages the contribution to grow.