If he is balking at eating more, smoothies are a good way to get more calories in. (kids are more willing to drink them if you call them "milkshakes"!). Add some jelly to that PB. There are also some supplements for kids to increase calories in picky eaters.
He might eat flavored cream cheese on celery or on apples. Just trying to think of extras you might add. Good luck and please update!
Some antibiotics can cause a false positive for urinary ketones, as can highly colored urine, sometimes UTI's. But it sounds like none of these are applicable in your childs situation.
Some children can have very high metabolisms. I wonder if your child simply needs more calories in than he is taking right now. Even though what you describe intake-wise is ok, perhaps he simply needs more than that. Is he very thin? Very active? (I'm sorry if you already said that, can't go back to see).
Try upping his caloric intake for a few days to see if the ketones disappear. That would probably be what I would do next. Plus monitor the occasional fasting BS and 2 hr post prandials. Keep records and make an appt with a pediatric endocrinologist if this persists. It's not an emergency, so ER care is not optimal, as you know. Their outlook is treat and street. Follow ups should be done by medical professionals who have both the time and expertise in this area.
Lorit,
I wish I had more to offer you, but all I can think of is dehydration. Does your son drink much water? You mention the foods he eats, but not whether he is well-hydrated.
Does he have allergies for example, that cause a runny nose? Is his urine dark or light? Has he lost any weight? Or has he maintained his weight while getting taller? Any recent physical "trauma" or injury that may have caused digestive problems?
Diabetes may not be a factor here, but as you know, it often doesn't "just happen suddenly." Parents can often look back over a sustained period of time and point out some symptoms that suddenly paint the picture.
As a nurse, you likely have thought of all this already. My advice is to TRUST your mother instincts and to push for either different tests or different doctors to help you get to the root of it. You might try random BG tests, in addition to the fastings, just to get pattern of how his body is handling the food he's eating. Keep those records to review with the doctor. The ER may not be the best place for a careful review of his history -- but your pediatrician or a pediatric endocrinologist will be able to help you interpret the results.
As you know ketones are passed into the urine before they build up in the blood (which is much more serious), so you are right to be investigating now.
Here's a website with some information on ketonuria; maybe something here will ring a bell for you: http://www.rnceus.com/ua/uaket.html
Good luck, Lorit!