It is not really advised to put Karo syrup in a baby bottle. If your child is constipated try "apple-prune" juice in the baby food section or dilute regular prune juice maybe ounce for ounce.
The use of Karo syrup for infant constipation is very effective. Yhe amount is the same no matter of the age of the child. It is two teaspoons for any size bottle of formula. Never use karo in water bottles, It is only used in formula bottles. This can be trpeated until results occur.
this is an old habit, and not a very good one. You want to leave sugar out of a child's diet for as long as you can unless it is natural sugar, that which is found in fruits. Otherwise you will cause habits that can lead to health problems.