My 18-month old stopped eating!?

It's very normal. My son has went through this at different ages, many many times. It can be very frustrating, but it will p. He is almost 5 and we are going through again right now. At 18 months he refused when he was getting more teeth in, you can check her gums and see if any seem to be breaking through her gums. Our son was a preemie, he weigh 3 lbs at birth. He is almost 5 and weighs 31 lbs. He has always been between 1 percentile to 10 percentile on weight. At one point he was on pediasure for 2 years. He is in the 10th percentile right now, his highest. We used to panic when he would go through his phases of not wanting to eat much, since he has always been so skinny, he looks like we starve him. He has always been a picky eater and from what I hear most children seem to be. His doctor has had our son on Periactin since he was 1 or 2 yrs old, for several yrs now. It is an antihistamine that is rarely used anymore for allergies, he is on it because it is supposed to make him feel more hungry thus he eats more, his pediatrician is trying to get him to gain weight. They have ran every test in the book on him and found nothing wrong with him. He is very active. Right now, we have to sit him down and tell him to eat, we try to give him the foods he likes. But 2 weeks ago he was eating like a little pig. Kids just go through so many different phases, we never know from one day to the next what its going to be like around here!