Birthmark tattoo

I want to get a tattoo in reference to my son but I don’t want something cheesy, I wanted to do something original and special. He has a red birthmark on his lower back right in the middle (the same place you’d put a tramp stamp) and I was wondering if it’d be weird to get a tattoo in the same shape and place as his birthmark? Maybe with his birth date underneath or something? I think it’d be really special but most people won’t get it as it’s just a random shape. I’m also worried that he might think it’s weird when he grows up especially because of the place that it’s in. I’m not set on the idea so no rude comments please, just wanted other people’s opinions on this. TIA!