There are a couple of possibilities for a problem like this. He may have laryngeal paralysis. This allows water, food, or saliva to go into the trachea, which causes the gagging. alternatively, he may have a heart problem. The poor circulation allows fluid to accumulate in the lungs causing the gagging. Remember, this website is not designed to replace your relationship with a veterinarian. There is no substitute for seeing and feeling the pet to determine the diagnosis, and treatment plans can be too complex to cover in their entirety in this venue. I am happy to share our experiences here at the Sanctuary and offer second opinions based on response to therapy, but please seek a live opinion first.