So, I don't know much about Danes. But, I do know quite a bit about food. I noticed on the picture of Dan that his nose looks dry and flaky. Maybe its dirty..but anyway...when Cabeza started having problems with food (gassy, tummy problems, loose pooh) he also had itchy skin and that dry flaky nose. We experimented with food and I eventually started feeding him raw. All the problems, including the dry flaky nose and itchy skin went away.
We don't feed raw anymore (we jus supplement with it) but I've got him on Wysong, and he still doesn't have any of those problems. You've already gotten great advice about food, but I just wanted to throw that in cuz I noticed Dane's nose and it looked just like Cabeza's did.
Get him off Science Diet, even if there is another explanation for the problem. Its junky. Like feeding your kids nothing but Corn flakes and bacon bits.