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Hello. Thanks for the question.
If he is overweight, then it's most likely dietary. He was taking in more calories than he was burning. The best way to get him down to a normal weight, is to put him on a restricted calorie food. You'll probably end up feeding ~ 1 cup, twice daily. This should allow him to lose weight over the next few months and get down to a normal weight.
Once at a normal weight, you increase the amount you are feeding, or you switch to a maintenance diet. In an older dog, I would go with something that has glucosamine and chondroitin in it. Iams and Eukanuba senior diets will have these. A veterinary prescription diet (J/D- joint diet) also has this.
I would also try to get him off the Prednisone unless it's absolutely necessary.
I hope this helps, let me know if you have any other questions.