Holding his tummy probably means it is hurting, I would take him to the doctor , he will need liquids to stop him dehydrating, try pedialyte, can you remember what he was eating prior to his throwing up ?
the first time he threw up some sort of pasta he ate at daycare. the second time he only had some milk. if it means anything when he threw up his milk it was in chunks, not liquid.
aha Perhaps it is a milk allergy ...I would run it by the doctor