You could have the lawyer require that your ex-son-in-law will refrain from permitting your grandson to be baptized until he reaches at least the age of majority. Insist that the Bible reports that Jesus did not get baptized until he was 30 years of age, therefore in order to follow his example, he should be an adult.
A 9 year old cannot vote, get married, enter a contract, consent to surgery, drive a car, join the army, etc. If he is not considered mature enough to make those decisions, he is also not mature enough to decide to be baptized.
The WTS recommends a long engagement period before marriage - with the goal of getting a clear picture of the person they intend to marry. They say that marriage is not something to rush into. How much more serious is the step of baptism? How much more seriously should the person weigh the belief system that they are considering joining?