This is something you should never say to a bilingual, ever. Doesn’t matter how interested you would be to hear something in a particular language. All bilinguals I know absolutely detest speaking “on demand”, just for the sake of it. Language is not about performance, it’s about communication. It’s like saying “Draw something!” to a painter, “Write something!” to an author or “Show me some moves!” to a dancer. We know we can speak the language and will use it wherever needed – but not to show off.
Keep this in mind with your children as well. No matter how proud you are about what languages they know – never ask them to say something in front of others just to prove they can. I would even recommend diffusing situations where another adult asks your little boy to speak something to show he knows a language. As I am sure all multilingual parents are already aware – it is bound to have happened at some point! – the result is just a refusal to speak at all. In such a situation your son may not even want to respond to you even though you would normally speak the requested language with each other. He is just staying resolute about not saying anything. And he has his full right to do so.
We don’t learn a language for the sake of it, the underlying motivation is communication, not performance. Languages should be a normal part of a bilingual child’s life, not something they have to show off or be nervous about. They should feel motivated to use their languages and enjoy the benefits they bring to them. Children learning languages can be very sensitive about their skills and afraid of making mistakes. If in addition they are also of a shy nature, asking them to utter something in a particular language is just not the right thing to do.
May the peace and power be with you.