Molzy, you said he was adopted then was fine for the FIRST TWO WEEKS..Then boarded and then developed the issue? So he is probably confused with his home and family? Sounds like anxiety, the changing of homes and medicines confuse the dog…could be some nausea/stomach acid but I think it might be psychological…
If all tests are passed and clear, the licking and swallowing can be for a variety of reasons; my golden does this before he sleeps, after he eats, when he wakes up, usually 3-4 swallows. Dogs do this to for some or all of the following 1.) calm down for sleep 2.) relieve nausea 3.)relieve stomach upsets/acid 4.)dry throat 5.) anxiety 6.)serious ailment or blockage. If its not a serious physical ailment then dogs are similar to humans once they pick up a habit that they can use to help cope with life they will continue it.
If they problem is from stomach acid and is diet related then try boiled rice, potatoes, with shredded chicken breast. Yams, celery, carrots, bananas and oatmeal seem to help too.
If the problem is psychological, meaning for example, my dog got carsick and threw up, and then within a couple of weeks was attacked by 2 dogs and had someone in the family get hospitalized as well. This left him with extremely high anxiety on what was going on. So perhaps, this type of licking, swallowing is way for him to cope. But taking him for long walks throughout the day has made the situation better; introducing him to new scents. Next, he is a retriever, so taking him to play fetch at the beach reminded him of who he is and it seems to have calmed his night issues as I have a feeling he is waking up with nightmares and does a lot of this gulping.
In short, physical activity is extremely important, they need to be active for hours throughout the day as opposed to traditionally just being in the house; this will make them stronger and settle their issues! If you need to hire a dog walker to help, you need to do it for the pooch! Good luck!
