Nope, it's not! Based on his photos you might assume that his tongue always hangs out, but that's just because I tend to upload photos where it's out. I normally ask him to stick out his tongue before I take a photo!
Although his tongue does hang out a lot of the time, there are large portions of the day where his tongue is inside his mouth and his underbite is clearly visible. Some internet trolls have told me that he has "hanging tongue syndrome" and I'm mean for thinking it's cute, but he doesn't have that. He is able to keep his tongue inside his mouth when he wants to, and while he's awake his tongue never gets dried out from being outside of his mouth for too long. The only time that ever happens is when he's sleeping. Also, after I taught him how to stick his tongue out as a trick, he started sticking his tongue out more often. So I think that he sometimes keeps his tongue out to get some treats!