Hi again, Sonja and thanks so much for your update on Topcat!
I'm glad the vet treated him for an allergy and it seems to have solved the problem!
Yes, being overweight in senior cats can lead to diabetes and other medical problems, so try to feed him a healthy, low carb canned food (grain-free, if you can) and decrease his serving sizes, slightly. Maine Coons ARE naturally a big breed, but even if you can get him to lose a couple of pounds, it will be healthier for him. Some well recommended grain free foods are Wellness, Blue Buffalo, Felidae, and others you can find in your pet supply store. Just read the labels and make sure real meat and/or fish (no by-products) is the first ingredient and it doesn't include any soy, gluten, wheat, corn, dairy products, artificial colors, flavors and/or preservatives. These are common allergens for cats.
Here is a great collection of articles written by a veterinarian on the best way to feed our cats (including special medical conditions): http://www.catinfo.org/
I hope Topcat continues to do well!