bubba, I've found them pretty true to size, my eldest is a size 12/13 depending on the shoe and she is wearing her size 12 boots from last year. I thought she might have grown out of them but I'm guessing they were a bit too big last year 
Is the wee fella just into a 12.5 at Clarks or has he been that size for a while? I only ask as I know that Clarks shoes actually fit up to a half size bigger than the size on the shoe, so a 12.5 shoe actually can fit a size 13 foot! HTH 