What Causes Skin Fold With Bleeding Cut In The Lower Back?

I noticed my 10 month old son has skin folded together on his butt/lower back (noticed it 3 months ago)and didn t know there was cause to pause. Then I was reading about tethered cord ect..This morning I took a better look at it and in doing so it looks cut open (about 1 inch long)and is bleeding. Sound I make an appointment with my ped. for a MRI or just wait til his next appointment and just mention it?