Maybe Callum or Callan and call him Callie for short

I would not know how to pronounce Caillou .

My DH is very sad that his name has dropped out of popularity. Your DS would be unique for his age group if you used his name! But I gotta say, I have never liked DH's name. I have struggled to pronounce it since I was a child (I have a cousin with the same name). And DH always laughs at me when I get it wrong.