GAH!

One thing I learned about how to check a dog's sex when your not to sure, and while it's not to glamorous, is feel. If someone flipping held him while another person ran a hand down his belly they'd know. It's not to hard and it's not like dog is going to be scarred for life by it. That shelter doesn't look good at all.
I'm assuming that he's to matted or fury for them to see visually.