Dog feeding and diet

[Dan2427]

I have 2 dogs that I got when they were puppies, they have always been able to eat whenever they please. One is 9 months old black lab and the other is 9 years old a min pin. Recently we watched a chocolate lab who is about 4 years old, and now is kind of ours the original owner doesn't want him back and he is a really well trained dog and gets along with my black lab really well, well enough that she gets depressed when he is gone. Just not the same dog and doesn't eat. Anyway there was some background info, but my real question is the adopted dog is/was on a diet he's 92lbs just a huge dog I wouldn't say he is fat or obese just a huge dog. Is there anyway I can get him back to eating whenever like my first two dogs? He was apparently on a water diet too because the first time he got water whenever he pleased he would drink so much and throw up just water after drinking so much. He is finally in the swing of drinking as he pleases. How can I get him to eat as he pleases without making him throw up/over eat? Reason being is my first two dogs don't eat barely anything when I feed them in the morning and night. I suppose when they get hungry enough they will get in this habit. But I'd like to just fill there the big 3 gallon automatic feeder and let them eat whenever. Thanks!