When it comes to feeding my boys for the day....I am sensitive and want to make sure they do not go hungry at school, just like you are worrying about. But, my boys are different....one wakes up starving, and my youngest wakes up not "feeling hungry" for about 45 minutes after he wakes up. This can be a problem....
I have learned to wake my youngest son up earlier to allow his appetite to kick-in in the morning. This means getting him up at 6:45....I let him lay in bed and watch TV until 7:25, then he will be dressed and ready to eat by 7:40....and, out the door by 8am. This allows us 6 minutes to get in the car late! Which is how long it can take 2 boys (some mornings) to walk to the car :O)
At most elementary school's they are pretty strict about not letting kids up and out of their chair when it's lunchtime. I mean, can you imagine allowing ALL 600+ students to wander during lunch? No way, so he probably DOESN'T have the problem at school because it's a "rule". a "timed rule". (At our school they allow Kindergartners 25 minutes to eat) The trick is to apply on the same "rule" at home...
He will need to learn that "life goes on" for everyone else in your family, and that the world doesn't revolve around him nor his appetite :O( But, he is only 5 yrs old, and this is probably his first year of school, so he's learning EVERYTHING for the very first time.
If it were my situation, I would try the 15 minute idea (with a backup morning plan ready). Without creating "drama' about the whole "15 minutes idea", tell him it's time to eat and give your usual reminders. When it's time to brush teeth, then clean up the plate without saying anything and ask him to brush his teeth. He will then either go brush his teeth OR say, "but I'm still hungry"....you can lovingly say, "I know honey, but it's time to go to school, please go brush your teeth" AND WALK AWAY! In the meantime, though, you've already put some dry cereal in as baggie in your purse for him to nibble on in the car on the way to school. This should be used as an emergency only....only if he gets in the car crying because he's still hungry....that always break my heart...
I would begin to work on this NOW because the Holidays are coming and school schedules are all mixed up. It will be a "dry run" (without too many consequences) until after Winter Break.....If he misses his breakfast, then just put extra food in his lunch to nibble on at recess if he gets hungry early. I don't even think you need to make a big deal out of the breakfast....if he eats, he eats....if he doesn't... he will eventually!
Anyway, I think this is pretty normal for kids to be distracted when they eat....when both of my boys were toddlers this was an every meal occurance, but now we've formed a working routine allowing everyone to eat :O)
I hope you are able to figure out some loving way to make this happen for your son...
~N. :O)