It's time for some diagnostics with your vet to determine the cause of the diarrhea. If he's been on the same food and none of the other cats are sick, he may be dealing with some bacteria-related issues in his gut. With parasites, we'd expect to see the other cats having issues, too, but that doesn't necessarily mean they aren't afflicted with something, as well. On top of this, devons are predisposed to inflammatory conditions in the GI tract like inflammatory bowel disease which may be part of the cause of the diarrhea.
To answer your question in short, he's probably having some issues making either into the box or he's overshooting his mark with the diarrhea. It may help to place a potty pad beneath the box to give more room to avoid messes or even the plastic tray from a dog kennel (these are easily hosed out and what I used with rescue cats when they're having diarrhea).
Your vet is likely going to recommend a fecal workup including parasite screening, PCR panel and fecal culture to check for anything that shouldn't be present such as parasites, protozoan, bacteria, etc. They will likely treat with GI antibiotics like flagyl or baytril and may also recommend a high fiber food to see if this helps with his stool quality. If it does not, please feel free to stay in contact with me and I will make some recommendations in the future. My career focus is working with GI complications in purebreds, so I would be happy to offer you continued assistance if it's needed.
I’ll be standing by if you have other questions. Let me know if I can help further.
Did my response help to answer all of the questions that you had? If questions remain, please reply and let me know how I can assist.
Before signing off today, please take the time to use the star rating system at the top of the page to leave a rating for me. Until this is done, the website will not compensate me for helping you.
If you would like to request me in the future for pet-related questions, you can do so by accessing this page: http://www.justanswer.com/pet/expert-pitrottmommy/?rpt=3800