Ermintrude, I am sorry but I find your attitude completely unreasonable. As grandparents it is absolutely nothing to do with us what our children decide to name their children. It was our choice when we came to name our own children, now it their time. It does not matter how outrageous the name may seem to you, we are a different generation and will have our own ideas. Don't you love your son unconditionally, don't you support his right to make his own decisions, don't you want the birth of his child to be the most amazingly, magical time? Don't you want just to share in the joy of his son's birth, don't you just feel love and pride?
Mostly though I just cannot understand why you would want to cause a rift at this, the most precious of times. Having a grandchild is the most wonderful thing in the world, and our 'job' is to support whatever our children decide, not just on a name but on any decision they make. I just cannot understand why you would fall out with your son at the most important time of his life. I do not know if you have other grandchildren, but if so surely you would not wish to cause a rift, how can you even think of anything but the overwhelming sense of love and joy and happiness that a grandchild brings. I think you could have created a problem for yourself - I think if I were your DS or DIL I would always remember how you, at what should be the most wonderful time when your heart should be bursting with pride and joy and happiness, had instead had a disagreement. It might seem unfair but I think you may have done great harm to your relationship. Maybe I am unreasonable myself, but if I were your DS or DIL I would always remember the fly in the ointment at the time of great joy, I think that underneath I would not properly be able to forgive.
Sorry, I just think whatever the name, you are being both totally unreasonable, and on your DS and DIL's behalf I feel angry. How can you possibly do this to them? I would suggest the real problem is the money you have lent, but even so I would be completely furious you created a disagreement at this time.
I would buy the largest bouquet of flowers and beg forgiveness for being totally and completely unfair, unreasonable and unkind.
Oh and btw I am looking after my precious baby grandchild today - and he has a girl's name! I can't possibly imagine him called anything else now.
Recommendations please, for a stopover on the way to Loch Tay