you have not mentioned the weight and height of your child so i can not comment on the health.but remember children eat by need and adults eat by habbit; and thinness is not bad ,what is important is activity or agility and frequency of falling sick ;so if your son eats less but is very active and does not need to meet a doctor frequently then on the whole he is healthy and you should not worry.
Next Steps
get a growth charting done for your child.
Health Tips
a child need approx 1.5 gms of protein per kilogram of body weight to grow so for a 10 kg child needs approx 15 gms per day and one egg has 6 gms of protein and a 100 ml glass of milk has 2-3 gms