I forgot to add, I would first go to your vet before starting an exercise program, and ask their opinion on whatever diet you have him on. Have them look him over, check his heart, lungs, etc.. I would personally have his thyroid checked, and blood sugar for diabetes. If he's all good then start your program and enjoy, and I would take him back to the vet periodically to make sure he's doing well on it. :)
Doggy dieting is one of my favorite things to help friends and clients whose pups are gettin chubby.. Here is my take..
Dog food companies have one purpose- To sell dog food. Whatever the label on the bag says it is most likely too much. And if you want him to lose weight you have to feed less than the recommended amount.
My rule of thumb is one measuring cup per 20lbs of dog. I have a 45lb pit bull and she eats 2 cups a day, plus a couple treats. An eighty pound beagle must be a sight- I would love a picture- is he full beagle?.. Your typical beagle should weigh 20-25 pounds..Even if yours is built bigger than average, I can't imagine a beagle over 40lbs that's not overweight, or part something else. If you want a 40lb dog you feed 2 cups a day, including treats. Of course, every dog is different so it can vary.
You can't forget the treats.. People come in and say the dog barely eats his food and is still fat, and then I ask how many treats/people food the dog gets and they blush...
-Break your treats in half or buy smaller ones. Cut down the amount of treats, or give him green beans instead- Many dogs love green beans and they are low cal.
-Give him no more than 2 cups a day of low fat food, and split that amount into two meals. Dogs that eat only once a day tend to pack on weight- their bodies go into storage mode, so feeding them in seperate meals helps lose the weight. And no people food!
-If you want to add bulk without the calories, add 1/2 can of green beans, it'll fill him up so he won't guilt trip you into giving him more food.
-Lots of exercise. If you don't already walk him every day, then start by doing short walks to the end of the driveway or street for a week, then farther every week til you do a mile or two. Exercise is just as important as food amounts, and its great for his mind and spirit too.
-When he gets down to a good size, you can play around with the amount, more if he's too skinny, less if he starts gaining. You can weigh him on your scale at home, just pick him up and weigh the both of you, then subtract your weight.
Hope this helped :)
If he was a self feeder, maybe you should think of monitoring his food intake and give him only what is recommended on the bag. They also do make diet food now for obesed dogs so try a bag of that and give it time.