Eating lots of food and not gaining weight is worrisome. You definitely need a second opinion. There are a number of things that can cause that. Has he been tested for heartworms? I notice you live in oklahoma and in a warm enviroment that's definitely a good possibility.
Has he had his blood sugar tested? That can cause excessive drinking and urination, not to mention weight loss. He is old and this is a good possibility.
He needs a full battery of tests done to determine what the issue was. I don't believe your current vet when he says there is nothing wrong. A dog eating a lot and not gaining weight, especially when not active, points to there being an issue.
I would not keep the poor guy in his crate all the time. This will only make his issues worse since he's not getting mental stimulation and exercise. Senior dogs often have issues with incontinence and crating all the time is not the answer. You may want to try a male dog diaper (it goes around the waist so they can't pee on stuff). That way you can let him out of his crate and he won't soil your floor.