You probably know this, but don't say "spit it out" or "cat got your tongue" or "you don't know your own name?". Nothing demoralizing, but you don't seem like the type to so nothing personal towards you. On seeing if he wants to speak more, just do what he feels most comfortable with. You can ask him a few yes/no questions to start just to be more used to participating, then go with one word answers after that. See if the comfort of speaking progresses, and go from there! Thanks for asking!