He is 6 months old. He poops while walking

As long as he can do it, what difference does it make? Yes, for a bulldog, that is normal. They don't actually squat like a female dog urinating. They get comfortable on their hind legs and just bend down.

My poodle used to do a handstand when he urinated.

As long as he's pooping, and not constipated, it's normal for him. The only time to be concerned would be if he's not pooping, or if you notice he's in pain while he's doing this.