My 12 months old boy since he start pediasure,he at least will vomit 1-2 times per day.Why is it like that?

Pls do check the portion measure you are giving...is it too concentrated maybe? Start with moderate quantities to check for allergies and dislikes and increase in quantity gradually.

You can try the above method but If a child has a reaction to the proteins in Pediasure (milk and soy), then this product is going to do more harm than good.

I would reduce the milk intake Aside from committing anything else?

Dilute the portion for a start and see if it improves.