If he doesn't have an appetite because he is sick, try giving him breastmilk/ formula instead. Don't drill on schedule at this point. When you are sick, you don't follow your regular schedule too, right? 
Don't worry too much about weight, every child is different. My daughter is turning 4 in December, she just started eating better after she turned 3 and finally reached 30 lbs this month. My 27 month old son is only 24 lbs, he just doesn't like to eat. His 9 month old brother is 22 lbs, this one loves to eat. They are all happy children so I'm not too concerned
"My mission in life is not merely to survive, but to thrive; and to do so with some passion, some compassion, some humor, and some style." -- Maya Angelou (1928-2014)