Is buclizine syrup (longifene) good for a kid? My 2.5yr old son's doc gave him coz he is not eating much. Is it safe?

No. Longifene is not approved for use in children. There are many reasons for a 2 1/2 year old not to "eat well". The first thing you want to check is your son's growth charts. If your child is not growing well according to the standard growth charts, further testing may need to be done. Also, many children this age don't eat well because they get too many calories from what they drink.
Growth curves? This class of drug is in the antihistamine group and in theory can be used as an appetite stimulant. This is definitely not routine care for a 2.5 yo in the us, and i would be very wary of using it. How is your child growing (weight and height) and developing? Does he really need this intervention? If he truly is not growing, i would want to try to find the cause.