My son LOVES smoothies. I know it doesn't sound like a good breakfast for the car, but we do it all the time. Even when he was your son's age. I just put the smoothie in the First Years Take and Toss straw cups. It is very difficult for my son to get the lid off (he did eventually learn how, but he has to use his teeth to get it off and it takes him a really long time, so I notice it and can say something before it's off.) Also, I find they don't spill easily, especially with the smoothies because they're thick.
I don't have a recipe as I just eye-ball everything, but below is an estimation of what I use:
1/2 - 3/4 cup apple juice
1 banana
1 cup frozen strawberries
1/2 cup frozen blueberries
3/4 cup plain yogurt
handful of spinach
1 Tablespoon wheat germ
Blend all in blender
Depending on how accurate my guess on measurements is, this should make about 3 of the take and toss cups. My son usually drinks 2 and I drink the other. I know it sounds wierd to put spinach in it, but you can hardly tell it's there and my son truly loves them, even with the spinach. He asks me for smoothies all day long. It's really easy and very healthy. Hope you enjoy it.