As your child grows older, you will encounter many different beliefs and fantasies and truths. For example, some families do not include Santa in any way in their Christmas observances, and some go to fantastic lengths and expense and time to spare their children from realizing that a jolly old man did not come down their chimney on Christmas eve for as long as possible. Some religions forbid the celebrations of birthdays, and some have elaborate birthday rituals and traditions. Your son will encounter these different, and valid, beliefs and will need to learn how to show respect for those who believe them.
Some children have experienced tremendous loss and neglect in homes that are full of hatred and anger, and others live in luxury without a care in the world. Other children live in a more moderate world, with a family who loves them but perhaps there is a tight budget, the death of an elderly grandparent, a car that isn't always reliable - the cares of this world that are realities.
People will rudely blurt things out that hurt, or innocently spoil a surprise. It just happens.
I encourage you to just focus on your son. If he's seen death and loss up close, then do what is necessary to help strengthen him and give him a solid foundation in his life. Provide security, truthfulness and love.
So what if he knows Mickey Mouse is a cartoon, and a guy dressed in a costume at a carefully orchestrated theme park that costs a ton of money to go to? Plan a fun, simple, magical evening with him. Make mouse ear cupcakes with him (mini Oreos to resemble ears, licorice strings cut to look like whiskers, with chocolate frosting, and eyes from a tube of white gel that they sell in the baking aisle). Watch the most fun Disney movie you can find, or see if they have the old Disney cartoons online. Sing the "Who's the Leader of the Band" song and have your own electric parade around the house with glowsticks and noisemakers (dry beans taped in paper cups or aluminum foil pans taped together make great tambourines and it's a fun craft). Make his life fun and secure and he'll know that his truth and his foundation and his joyful times come from family, and it doesn't matter if a cartoon is real or not, or if the Keebler elves make those cookies or if a sponge can wear pants and live in a pineapple. Show him the balance between realizing what's not true and what is, and how to have fun with the crazy stuff (mutant ninja turtles who like pizza!) and how to depend on the real stuff (his family loves him and his home is secure).