Burton,
I wouldn't suggest the Wolf King Solid Gold because it's for large breeds and boxers are a medium breed dog. I use the Solid Gold Hund-N-Flocken, which is the lamb and rice, my boxers somewhat had the same problem with switching over it took a few days for there stool to be completely firm. They both love the Hund-N-Flocken. I'm not saying start to switch him again, but like I said boxers are a medium breed. So you might want to consider it once his stool firms up and if it doesn't then you can try switching to the Hund-N-Flocken and see if that changes anything. Hope it all works out for you.