Hi carla ;
welcome to HealthcareMagic
is an anti-inflammatory analgesic
and is even used in children ;the dosage is 10-15 mg /kg of body weight so if your son has taken half of 400 mg it is around 200 mg which according to his age or weight will be double the dose.If he is not asthmatic;or allergic to ibuprofen then i guess he will not have any major problem and may just have vomiting
due to drug induced gastritis
.If he is allergic then he may suffer bronchospasm
then you need to go to a ER.Presently without panicking please consult a doctor close by and keep a watch on your baby hopefully he should not have any major problem.