Dream Whip?

I've used Dream Whip and it works. It also tastes very good. Because I live in a very hot and humid climate, I always stabilize my whipped cream, no matter what I use (Bettercreme, Dream Whip, etc.), except for Cool Whip and the pre-whipped  Bettercreme, which come already pretty much stabilized. For stabilizing, I usually add about a tablespoon of gelatin (dissolved and bloomed in 2-3 tablespoons of water and then melted in the microwave) to every pint (2 cups) of cream, measured before beating. I drizzle it in once the cream is full whipped. At least where I live, you have to refrigerate it anyway once it's made and used. After the gelatin gels, you can leave it out for a while, but you should put it back the leftovers back in the fridge after they're served, more because cream is perishable than because you want it to keep holding its shape. Does that help any?

No problem at all for that amount of time. Once you ice it and put it back in the fridge, the stabilized cream will set and hold its shape (for days, even, but I'm sure it would go bad long before that!).

I'm sure your cake will be a masterpiece, and of course your son will love it because it's just what he wanted. If you haven't made gelatin bubbles before, good luck with them. Once you get the hang of them, they're easy, but I found them very difficult to master (it might have just been me, of course!). My best advice for them is to anchor the balloons  to toothpicks with duct tape before dipping them and then sticking the toothpicks in styrofoam (spaced pretty far apart) to dry. They're well worth the effort, though.