Nurture not destroy
Should we tell our kids the truth or just give them the freedom to believe what they will?
While I understand about wanting to protect your litte gem from the possible hurts of following his dreams, one also needs to remember that it we often regret more the things we didn't do, rather than those we did. Yes, he may fail. That is part of life, and it is through our failures that we learn where to improve. At least he will know that he tried. And more than that... he'll know that his mum believed in him enough to allow him to try. And that she loved him enough to still be there when it all fell to pieces (if it does!).
My 5-year old daughter has decided that she'll be a mermaid when she grows up. Her exact words... "I'm going to be a mermaid when I grow up." I've chosen to allow her to nurture that dream rather than destroy her amazing imagination with the facts. Besides, by the time she learns that mermaids don't exist she will have most likely decided to be something else anyway. An octopus perhaps. Who knows?
Nurture their dreams, it will teach them to respect your expectations and hopes for them in the future.