My son (5 1/2) is not much of a breakfast eater, either, never has been, not even as an infant! Seems to run in the family. My brother and I are the same way. I can't eat until I've been up close to an hour after I've awakened.
My son started Kindergarten last year and my concern was the same as yours. I watch him, and for him to eat a large breakfast makes him feel physically ill (this is the same outcome I have as well as my brother, which is why I think maybe it's some genetic proclivity?)
One thing I do is to give him 2 or 3 choices the night before for what he wants to eat for breakfast the next morning. In the morning, if he balks at it, I remind him that is what he chose the night before.
I think something like a power bar is just fine...gives him nutrition and a start for the day. I usually give my son really simple, SMALL, and nutritious choices for breakfast. A small bowl of cereal without milk (I buy the cereals that are made with whole grain, but have add something sweet to it like raisins, blueberries, strawberries, cherries, dried cranberries, etc.), a small cup of yogurt or bottle of drinkable yogurt, a small wholegrain blueberry waffle "cut like a pizza", french toast sticks that he can dip in a little syrup, a Special K bar, etc. He also has to drink milk or orange juice.
We live a few blocks from his school, so I also had as a rule that if he didn't eat his breakfast (or took too long eating it such that he would be late), we had to RUN to school instead of getting to take our time walking leisurely together. I'd make him do sprints the entire way. That happened a few times, then he got the picture and started to eat his breakfast so not to have to arrive sweaty and out of breath to school. Ha! (Hey, it gave me a good workout to since I had to run alongside him!). :-D
Good luck and hope something works for you!
Sincerely,
S.