Shoe sizes HELP!?

I have never had an issue with shoes… up until now. My 6 year old needs new winter boots but they don’t sell them locally (which is odd giving that we live in Minnesota and it snow a lot.) They have toddlers and adults. But none for around his age so I’m ordering them online. Now I’m seeing toddler, little kid, and big kid sizes and that’s confusing me. He’s a size 5-6 now in his shoes but when I look at the little kid sizes, they don’t have a 5 in little kid, and the big kid sizes do, but those are definitely too big for him (the size 5 in big kids are for feet as big as mine) I don’t know his exact length so it was important that I just find size 5 and not all this extra labels for it. All I know is he’s definitely not got 9inch long feet (size 5 in big kids) i don’t know what size I need!!