I wouldn't have thought this was underweight - or definately not to the extent the doctor is concerned about. My son is nearly 7, his weight fluctuates between 23 - 24 kg. He looks to me to be about average compared to all his classmates, so I wouldnt have thought 22.5 kg was something to be concerned about.
If it was me I would keep up with the 5 meals a day he is currently eating and keep it healthy - maybe double check with your regular doctor when you see him next.
Mr J (April 2005) Miss Z (Feb 2007) and Miss O (Oct 2010)