Also, it is interesting that, whilst most JWs criticize celebrating birthdays, they have no trouble celebrating wedding anniversaries which are simply "birthdays" of the wedding day.
Hi J-DUBBED, As other members have expressed, focus on improving your relationship with your son and his wife first. May be in a couple of years, you could convince your son and his wife to allow you and your wife to give your grand-daughter a present on the day before or after her birthday, or some other specifically agreed to day. There is no hurry. The WTBTS's Armageddon isn't going to happen.
If you want to plant a seed of doubt in your son using birthdays, I would say something like the following when your son's authentic persona is more dominate than his cult persona (i.e., a couple of months before or after assemblies/conventions and may be on a two week vacation from a WTBTS meeting):
Son, your mother and I would like to give our grand-daughter presents on her special day to make her feel special. We understand that as a JW you no longer believe that your daughter should celebrate birthdays by giving her presents. Where in the Bible is it written that birthdays should not be celebrated?
[Wait for your son's response.]
Since only bad things happened to two individuals on a Pharaoh's or King's birthday (i.e., Ge 40:20, Mt 14:6, and Mk 6:21) in the Bible, why does the Watchtower teach JWs that God does not want JWs to celebrate their birthdays? Are men adding to God's teachings?
[Wait for your son's response]
Wasn't Jesus Christ's birth celebrated by angels (Luke 2)? Why do Jews celebrate birthdays, and bar mitzvahs and bat barmizhs?
[Wait for your son's response]
What does the bible say in Deu 4:1-2 and Mt 15:1-10 about adding to or subtracting from God's commandments? Quickly and subtly change subjects.
Peace be with you and everyone, who you love,
Robert
P.S. - Learn to use https://www.biblegateway.com/ to search different bibles for specific keywords/phrases. I prefer the NIV Bible.