Other things can make ketones appear in the urine. When your nephew gets a blood test have them do an a1c (that gives an idea of blood sugars over a three-month period).
Personally, I'd hustle him to the doc now. But then, I have a ds who is type 1 and thankfully he takes good care of himself.
If this is type 1 diabetes, feel free to contact me...I'll help all I can.
Misti