Aye and he's a charmer ain't he? There are a number of approaches that come to mind. Theodore Roosevelt wore DRESSES as a child, was no uncommon thing in those days. Not that long ago, me beloved late Mum just SCANDALIZED the University of Houston when she wore TROUSERS to attend graduate level classes in geology. A WOMAN! In PANTS! Surely she must be . . . well, you know, not quite normal. You could point out that pants were really only even thought of for purposes of riding horses, and that before that unbifurcated garments were the norm. You could tell her that, while she is certainly entitled to her opinion, is it really helping her grandson to mock how he's dressed? Oh and aye, you could mention how much easier it is to change the diapers when a lad's in a kilt, vice trews! Oh and by the way---your son is a charmer indeed, as I mentioned already, but his outfit is simply smashing. His sartorial presentation rivals that of esteemed members Panache and Hamish, and I consider that dang high praise indeed! Best of luck to you, my brother, and I think, based just on the picture of your son, that you've got a darned fine start on things.