I would not worry about any reward/tantrum connection here. A 'tantrum' at his age (my son is Alex too!), due to hunger and crankiness, is not the same as a 4 year old throwing a fit because they want candy. At that age, it is just impatience from hunger/tiredness and a delay in food will create an inevitable upset. If he didn't want the balloon but chowed down the ice cream, then I would write that off as hunger.
In the future, as said above, I would simply say "no ice cream, thank you".