You may want to try feeding him only twice per day (cause he is an adult) and don't give him unlimited water. The more you give the more he will eat and drink and go to the bathroom. Fill his water bowl a lil here and there but make sure he gets water. If he is eating and drinking he will be healthy. He doesnt need non stop food to put on weight. He will put it on lil by lil. No hurry. You may want to try some puppy pads (even though he is not a puppy) this may help out with him having to go outside to do his buisiness so much. Good luck with the pup! Hope this helps. :D
