Do you know what? This makes me really cross!
Being able to "cry" on cue is nothing to do with being a good actor. This instruction is absolutely concentrating on the wrong thing it seems to me. I think it's just the WRONG instruction and is making you and your son concentrate on the wrong thing.
An actor needs to be able to inhabit the character required and react and act appropriately to the situation, but that need not mean "crying". Most of the time in life, we try NOT to cry (though tears may come in spite of this) and witnessing this is far more moving than someone bawling their eyes out. In fact, the only time I can think that a human being "tries" to cry is when they are being totally false (like a toddler trying to get something they want or an adult trying to manipulate another) and generally we can tell really easily that this IS false. Hence, if you ask an actor to "cry", without allowing them to act/react honestly and truthfully, you are unlikely to get a good performance out of them.
I cannot, for the life of me, understand the preoccupation with producing tears! If all the circumstances are right and the actor is "in the moment", then they will bring the truth of that moment to the acting. It will probably be deeply affecting, if it's sad, but the tears are irrelevant!
The director/casting director/agent - or whoever it is - should be looking for a truthful performance that moves the watcher to tears... it shouldn't have anything to do with whether or not the actor cries!! I would suggest ignoring the instruction (honestly, this is what I would do) and just going for the truth of the situation - if a child does this well, I BET everyone will be more impressed than someone being able to produce moisture at will! Tears may come, if they don't, it can still be deeply affecting. Just say "they want to see you being sad or frightened or lonely - can you imagine what that would be like? Think about a time when you've felt a bit sad or frightened or a bit lonely and get a bit of that feeling in your tummy". He'll know what "sad" is.
Good luck!
Take a look at this early interview with Meryl Streep (in my view, one of the world's finest film actors) - 6 minutes in - "I can't cry unless I'm moved - sometimes it just doesn't happen".
http://www.youtube.com/watch?v=bwszlmS3 ... re=related
Grr! "We want him to be able to cry" - how ridiculous!
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If they need tears, they can wave an onion under his nose.