While I'm waiting for your reply, I'm going to begin typing some general thoughts.
It's not unusual for dogs to act like they are just starving. Some dogs are much more food driven than others. For the dogs that are quite food driven, they can act absolutely crazy about food. I know this from personal experience, as I have a beagle who has, for all of his life, done exactly as you described - constantly begging and searching for food, getting into everything, and generally acting as though he is starving. I swear, he could subsist on air alone, haha. This in itself isn't abnormal. While there are some hormonal issues like the thyroid and cushing's syndrome that can cause a dog to have difficulty losing weight, in many cases it's just a matter of overcoming their naturally slow metabolism.
It's important that the vet has also checked for diabetes and liver enzyme elevations that could indicate cushing's disease. Typically cushing's does have other symptoms like excessive thirst and excessive urinations, but it's not unreasonable to go down that path if the ALP is high on lab work and he's acting hungry all the time. If they did just a total T4 (which is a common screening test for hypothyroidism) and it was in the low end of the reference range, it's also a good idea to send out a larger thyroid panel to an outside lab for a more clear picture of thyroid function. If his full lab work is normal, then it's likely his variation of normal to be as he is. With the episode of unexpected urination you described, it's also not a bad idea to have a urine sample checked for a urinary tract infection. (I just got your new replies and see that this has been done - very good!)
I've had good success with both Purina OM and Hill's metabolic prescription diets, but with some dogs we also have to add a significant amount of exercise to get the weight off as well. If diet alone isn't doing it, we have to reassess their exercise level and check for "cheating", if you get my drift! I've had many a frustrating client situation in which not everyone in the home was on the same page when it came to feeding and I had pets not losing weight because "grandma" or someone else in the home was cheating on the diet. A good way to manage treats and feeding in a home with multiple family members is to pre-portion out the treats and food each morning. Once the food and treats are gone, there's no more cheating by giving anything that wasn't set out beforehand.
Keep in mind that there does come a point at which we are restricting the diet too much, which causes their metabolism to slow to a crawl - they go into "hoarding" mode and then it's very difficult to get the weight off. It's important that we provide his metabolism with enough fuel to keep burning, which is why it seems like you are feeding him a lot on the prescription diets. It's the same concept as how we diet as people - eating a small meal every two hours. It might be that at 3 cups of food a day, he's not going to get enough fuel to keep his metabolism cranking.
As to your last comment that he seems that he's always hungry - some dogs absolutely are that way. My 25 pound beagle could put away 5 pounds of food - he's done it before when I accidentally left a bag of cat food out. I only feed him 1/3 of a cup of dog food twice a day and he follows my small children around licking up every crumb they might happen to drop. You may have already heard these tricks, but adding bully fiber rich foods like carrots, green beans, or canned pumpkin can help fill him up at meal times. Soaking the dry food in water before feeding him also will help him feel more full after eating.
I hope that this is helpful - what other questions can I answer for you?