Support his jaw with your thumb and forefinger. Sit down and position his body so that his head is lower than his chest and use the heel of your hand to deliver five firm back blows between his shoulder blades.

If it doesn't dislodge the item, then carefully turn the child over, keeping his head below his chest and deliver chest thrusts. Press down with two or three fingers in the center of the chest just below the nipples five times, just enough to depress the chest an inch or so.

Alternate chest thrusts and back blows until the object is forced out or the baby cries or coughs on his own.  You may need to perform CPR if it doesn't work.

Becoming certified in infant and child CPR is a great idea for parents. You can contact your local Red Cross for classes.