hey ambah,
i've seen quite alot of this happening esspecially when kids are meant to be sleeping or when their parents were leaving them at the child care. They do it to get your attention and make you feel bad!! They are quick to figure out the things that make mummy pick them up and cuddle them and give them attention, so they think if they make themselves gag and/or vomit then you'lll rush to them and they'll get their own way!
I've see kids under one do it, up until preschool/school age. We had one little girl that would do it ever morning until she threw up at the child care centre when her mum was leaving, to the point that we would take her straight to the sink as soon as mum left. She was absolutely fine five minutes later and would have a lovely day. The advice layla gave is exactly what we were told. Ignore the behaviour unless he makes himself physically sick, then clean it up quickly and put him back down. He wont do himself any harm! Good luck with it all