Gelding is definitely a must to make a safe child's riding pony. The standard timing is before the foal is a year old.
Please also have the foal handled and trained by a professional. You want your grandson to have a good and safe experience and you want the pony to be the kind that would be welcome in anyone's barn.
Please also have the foal handled and trained by a professional. You want your grandson to have a good and safe experience and you want the pony to be the kind that would be welcome in anyone's barn.