How do you help a 19 month old child who is quite gassy?

Investigate. Kids swallow some air with feeds and some gas is expected. However, some kids are unable to digest the sugar (lactose) in milk, and the gut germs make it into extra gas. Other foods may have a similar effect. Eliminating a suspected food for 2 weeks should improve the problem if it is a problem food.Keeping a diary of the foods you try, which make him gassy or not, should help you.
Diet Trial. Need to determine the cause. Usually it is dietary, so an elimination trial will be effective. These are slow so patience is a virture!